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|Rifle, 5.56 mm, M16|
From top to bottom: M16A1, M16A2, M4A1, M16A4
|Place of origin||United States of America|
|Used by||See Users|
Laotian Civil War
Cambodian Civil War
Invasion of Grenada
Invasion of Panama
Somali Civil War
Operation Deny Flight
Operation Joint Endeavor
War in Afghanistan
|Designer||Eugene Stoner and L. James Sullivan|
|Number built||~10 million|
|Weight||7.18 lb (3.26 kg) (unloaded)
8.79 lb (4.0 kg) (loaded)
|Length||39.5 in (1,000 mm)|
|Barrel length||20 in (508 mm)|
|Action||Gas-operated, rotating bolt (direct impingement)|
|Rate of fire||12–15 rounds/min sustained
45–60 rounds/min semi-automatic
700–950 rounds/min cyclic
|Muzzle velocity||3,110 ft/s (948 m/s)|
|Effective range||550 meters (point target)
800 meters (area target)
|Feed system||20-round box magazine:
(0.211 lb [96 grams] empty / 0.738 lb [335 g] full)
30-round box magazine:
(0.257 lb [117 g] empty / 1.06 lb [483 g] full)
Beta C-Mag 100-round double-lobed drum:
(2.2 lb [1 kg] empty / 4.81 lb [2.19 kg] full)
The M16 rifle, officially designated Rifle, Caliber 5.56 mm, M16, is the United States military version of the AR-15 rifle. The rifle was adapted for semi-automatic, three-round burst, and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO cartridge. The rifle entered United States Army service and was deployed for jungle warfare operations in South Vietnam in 1963, becoming the U.S. military's standard service rifle of the Vietnam War by 1969, replacing the M14 rifle in that role. The U.S. Army retained the M14 in CONUS, Europe, and South Korea until 1970. Since the Vietnam War, the M16 rifle family has been the primary service rifle of the U.S. armed forces.
The M16 has also been widely adopted by other militaries around the world. Total worldwide production of M16s has been approximately 8 million, making it the most-produced firearm of its caliber. In 2010, the M16 began to be phased out in the U.S. Army and is being supplemented by the M4 carbine, which is itself a shortened derivative of the M16A2.
The M16 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed assault rifle, with a rotating bolt, actuated by direct impingement gas operation. The rifle is made of steel, 7075 aluminum alloy, composite plastics and polymer materials.
In the early 1960s, the U.S. Air Force's rifle, the M16, and the Army's XM16E1, were the first versions of the M16 rifle fielded. Soon, the U.S. Army standardized an upgrade of the XM16E1 as the M16A1 rifle, an M16 with a forward assist feature and other improvements requested by the Army. All of the early versions were chambered to fire the M193/M196 cartridge in the semi-automatic and the automatic firing modes. This occurred in the early 1960s, with the Army issuing it in late 1964. Commercial AR-15s were first issued to Special Forces troops in spring of 1964.
The M16A2 rifle entered service in the 1980s, being ordered in large scale by 1987, chambered to fire the standard NATO cartridge, the Belgian-designed M855/M856 cartridge. The M16A2 is a select-fire rifle (semi-automatic fire, three-round-burst fire) incorporating design elements requested by the Marine Corps: an adjustable, windage rear-sight; a stock 5⁄8 inches (15.9 mm) longer; heavier barrel; case deflector for left-hand shooters; and cylindrical handguards. The fire mode selector is on the receiver's left side.
The M16A3 rifle is an M16A2 rifle with an M16A1's fire control group (semi-automatic fire, automatic fire) that is used only by the U.S. Navy and U.S. Coast Guard. The Coast Guard replaced their upper receivers with A2 receivers in the mid 90's, but retained the full auto capability by mounting the new A2 upper receiver on the old full auto lower receiver.
The M16A4 rifle was standard issue for the United States Marine Corps in Operation Iraqi Freedom after 2004; it replaced the M16A2 in front line units. In the U.S. Army, the M16A2 rifle is being supplemented with two rifle models, the M16A4 and the M4 carbine, as the standard issue assault rifle. The M16A4 has a flat-top receiver developed for the M4 carbine, a handguard with four Picatinny rails for mounting a sight, laser, night vision device, forward handgrip, removable handle, or a flashlight.
The M16 rifle is principally manufactured by Colt and Fabrique Nationale de Herstal (under a U.S. military contract since 1988 by FNH-USA; currently in production since 1991, primarily M16A2, A3, and A4), with variants made elsewhere in the world. Versions for the U.S. military have also been made by H & R Firearms General Motors Hydramatic Division and most recently by Sabre Defence. Semi-automatic versions of the AR-15 are popular recreational shooting rifles, with versions manufactured by other small and large manufacturers in the U.S.
5.56 mm cartridge 
The 5.56x45mm cartridge originally developed by Armalite had many advantages over the 7.62x51mm NATO round used in the M14 rifle. Most of these reasons were due to the dense and humid jungle in which US soldiers were fighting during the Vietnam War. The 5.56 mm cartridge was developed as a more medium range alternative to the larger caliber round used in M14 and M1A rifles; each soldier can carry more ammunition and engage enemy targets more effectively at shorter distances. It was also found useful in automatic and burst fire, developing the M16 into variants such as the M16A3 and current M16A4.
ArmaLite sold its rights to the AR-15 to Colt in 1959. The AR-15 was first adopted in 1962 by the United States Air Force, ultimately receiving the designation M16. The U.S. Army began to field the XM16E1 en masse in 1965 with most of them going to the Republic of Vietnam, and the newly organized and experimental Airmobile Divisions, the 1st Air Cavalry Division in particular. The U.S. Marine Corps in South Vietnam also experimented with the M16 rifle in combat during this period. The XM16E1 was standardized as the M16A1 in 1967. This version remained the primary infantry rifle of U.S. forces in South Vietnam until the end of the war in 1973, and remained with all U.S. military ground forces after it had replaced the M14 service rifle in 1970 in CONUS, Europe (Germany), and South Korea; when it was supplemented by the M16A2. During the early 1980s a roughly standardized load for this ammunition was adopted throughout NATO (see: 5.56×45mm NATO).
The M16A3 is a fully automatic variant of the M16A2, issued within the United States Navy. The M16A2 is currently being supplemented by the M16A4, which incorporates the flattop receiver unit developed for the M4 carbine, and Picatinny rail system. M16A2s are still in stock with the U.S. Army and Marine Corps, but are used primarily by reserve and National Guard units as well as by the U.S. Air Force.
The M16 rifle design, including variant or modified version of it such as the Armalite/Colt AR-15 series, AAI M15 rifle; AP74; EAC J-15; SGW XM15A; any 22-caliber rimfire variant, including the Mitchell M16A-1/22, Mitchell M16/22, Mitchell CAR-15/22, and AP74 Auto Rifle, is a prohibited and restricted weapon in Canada.
The first issues of the rifle generated considerable controversy because the gun suffered from a jamming flaw known as “failure to extract,” which meant that a spent cartridge case remained lodged in the chamber after a bullet flew out the muzzle. According to a congressional report, the jamming was caused primarily by a change in gunpowder that was done without adequate testing and reflected a decision for which the safety of soldiers was a secondary consideration. Due to the issue, reports of soldiers being wounded were directly linked to the M16, which many soldiers felt was unreliable compared to its precursor, the M14, which used stick powder, varying from the M16's utilization of ball powder.
Project SALVO 
In 1948, the U.S. Army organized the civilian Operations Research Office, mirroring similar operations research organizations in the United Kingdom. One of their first efforts, Project ALCLAD, studied body armor and the conclusion was that they would need to know more about battlefield injuries in order to make reasonable suggestions. Over 3 million battlefield reports from World War I and World War II were analyzed and over the next few years they released a series of reports on their findings.
The conclusion was that most combat takes place at short range. In a highly mobile war, combat teams ran into each other largely by surprise; and the team with the higher firepower tended to win. They also found that the chance of being hit in combat was essentially random; accurate "aiming" made little difference because the targets no longer sat still. The number one predictor of casualties was the total number of bullets fired. Other studies of behavior in battle revealed that many U.S. infantrymen (as many as two-thirds) never actually fired their rifles in combat. By contrast, soldiers armed with rapid fire weapons were much more likely to have fired their weapons in battle. These conclusions suggested that infantry should be equipped with a fully automatic rifle of some sort in order to increase the actual firepower of regular soldiers. It was also clear, however, that such weapons dramatically increased ammunition use and in order for a rifleman to be able to carry enough ammunition for a firefight he would have to carry something much lighter.
Existing rifles met none of these criteria. Although it appeared the new 7.62 mm T44 (precursor to the M14) would increase the rate of fire, its heavy 7.62 mm NATO cartridge made carrying significant quantities of ammunition difficult. Moreover, the length and weight of the weapon made it unsuitable for short range combat situations often found in jungle and urban combat or mechanized warfare, where a smaller and lighter weapon could be brought to bear faster.
These efforts were noticed by Colonel René Studler, U.S. Army Ordnance's Chief of Small Arms Research and Development. Col. Studler asked the Aberdeen Proving Ground to submit a report on the smaller caliber weapons. A team led by Donald Hall, director of program development at Aberdeen, reported that a .22 inch (5.56 mm) round fired at a higher velocity would have performance equal to larger rounds in most combat. With the higher rate of fire possible due to lower recoil it was likely such a weapon would inflict more casualties on the enemy. His team members, notably William C. Davis, Jr. and Gerald A. Gustafson, started development of a series of experimental .22 (5.56 mm) cartridges. In 1955, their request for further funding was denied.
A new study, Project SALVO, was set up to try to find a weapon design suited to real-world combat. Running between 1953 and 1957 in two phases, SALVO eventually suggested that a weapon firing four rounds into a 20-inch (508 mm) area would double the hit probability of existing semi-automatic weapons.
In the second phase, SALVO II, several experimental weapons concepts were tested. Irwin Barr of AAI Corporation introduced a series of flechette weapons, starting with a shotgun shell containing 32 darts and ending with single-round flechette "rifles". Winchester and Springfield Armory offered multiple barrel weapons, while ORO's own design used two .22, .25 or .27 caliber bullets loaded into a single .308 Winchester or .30-06 cartridge.
Eugene Stoner 
Meanwhile testing of the 7.62 mm T44 continued, and Fabrique Nationale also submitted their new FN FAL via the American firm Harrington & Richardson as the T48. The T44 was selected as the new battle rifle for the U.S. Army (rechristened the M14) despite a strong showing by the T48.
In 1954, Eugene Stoner of the newly formed ArmaLite helped develop the 7.62 mm AR-10. Springfield's T44 and similar entries were conventional rifles using wood for the "furniture" and otherwise built entirely of steel using mostly forged and machined parts. ArmaLite was founded specifically to bring the latest in designs and alloys to firearms design, and Stoner felt he could easily beat the other offerings.
The AR-10's receiver was made of forged and milled aluminum alloy instead of steel. The barrel was mated to the receiver by a separate hardened steel extension to which the bolt locked. This allowed a lightweight aluminum receiver to be used while still maintaining a steel-on-steel lockup. The bolt was operated by high-pressure combustion gases taken from a hole in the middle of the barrel directly through a tube above the barrel to a cylinder created in the bolt carrier with the bolt carrier itself acting as a piston. Traditional rifles located this cylinder and piston close to the gas vent. The stock and grips were made of a glass-reinforced plastic shell over a rigid foam plastic core. The muzzle brake was fabricated from titanium. Over Stoner's objections, various experimental composite and 'Sullaloy' aluminum barrels were fitted to some AR-10 prototypes by ArmaLite's president, George Sullivan. The Sullaloy barrel was made entirely of heat-treated aluminum, while the composite barrels used aluminum extruded over a thin stainless steel liner.
Meanwhile, the layout of the weapon itself was also somewhat different. Previous designs generally placed the sights directly on the barrel, using a bend in the stock to align the sights at eye level while transferring the recoil down to the shoulder. This meant that the weapon tended to rise when fired, making it very difficult to control during fully automatic fire. The ArmaLite team used a solution previously used on weapons such as the German FG 42 and Johnson light machine gun; they located the barrel in line with the stock, well below eye level, and raised the sights to eye level. The rear sight was built into a carrying handle over the receiver.
Despite being over 2 lb (0.91 kg) lighter than the competition, the AR-10 offered significantly greater accuracy and recoil control. Two prototype rifles were delivered to the U.S. Army's Springfield Armory for testing late in 1956. At this time, the U.S. armed forces were already two years into a service rifle evaluation program, and the AR-10 was a newcomer with respect to older, more fully developed designs. Over Stoner's continued objections, George Sullivan had insisted that both prototypes be fitted with composite aluminum/steel barrels. Shortly after a composite barrel burst on one prototype in 1957, the AR-10 was rejected. The AR-10 was later produced by a Dutch firm, Artillerie Inrichtingen, and saw limited but successful military service with several foreign nations such as Sudan, Guatemala, and Portugal. Portugal deployed a number of AR-10s for use by its airborne (Caçadores Pára-quedista) battalions, and the rifle saw considerable combat service in Portugal's counter-insurgency campaigns in Angola and Mozambique. Some AR-10 rifles were still in service with airborne forces serving during the withdrawal from Portuguese Timor in 1975.
In 1957, a copy of Gustafson's funding request from 1955 found its way into the hands of General Willard G. Wyman, commander of the U.S. Continental Army Command. He immediately put together a team to develop a .223 caliber (5.56 mm) weapon for testing. Their finalized request called for a select-fire weapon of 6 pounds (2.7 kg) when loaded with 20 rounds of ammunition. The bullet had to penetrate a standard U.S. steel helmet, body armor, or a steel plate of 0.135 inches (3.4 mm) and retain a velocity in excess of the speed of sound at 500 yards (460 m), while equaling or exceeding the "wounding" ability of the .30 Carbine.
Wyman had seen the AR-10 in an earlier demonstration, and impressed by its performance he personally suggested that ArmaLite enter a weapon for testing using a 5.56 mm cartridge designed by Winchester. Their first design, using conventional layout and wooden furniture, proved to be too light. When combined with a conventional stock, recoil was excessive in fully automatic fire. Their second design was simply a scaled-down AR-10, and immediately proved much more controllable. Winchester entered the LMR, a design based loosely on their M1 carbine, and Earle Harvey of Springfield attempted to enter a design, but was overruled by his superiors at Springfield, who refused to divert resources from the T44. In the end, ArmaLite's AR-15 had no competition. The lighter round allowed the rifle to be scaled down, and was smaller and lighter than the previous AR-10. The AR-15 weighed only around 5.5 pounds (2.5 kg) empty, and 6 pounds (2.7 kg) loaded (with a 20 round magazine).
During testing in March 1958, rainwater caused the barrels of both the ArmaLite and Winchester rifles to burst, causing the Army to once again press for a larger round, this time at 0.258 in (6.6 mm). Nevertheless, they suggested continued testing for cold-weather suitability in Alaska. Stoner was later asked to fly in to replace several parts, and when he arrived he found the rifles had been improperly reassembled. When he returned he was surprised to learn that they too had rejected the design even before he had arrived; their report also endorsed the 0.258 in (6.6 mm) round. After reading these reports, General Maxwell Taylor became dead-set against the design, and pressed for continued production of the M14.
Not all the reports were negative. In a series of mock-combat situations testing the AR-15, M14 and AK-47, the Army found that the AR-15's small size and light weight allowed it to be brought to bear much more quickly, just as CONARC had suggested. Their final conclusion was that an 8-man team equipped with the AR-15 would have the same firepower as a current 11-man team armed with the M14. U.S. troops were able to carry more than twice as much 5.56×45mm ammunition as 7.62x51mm for the same weight, which would allow them a greater advantage against a typical NVA unit armed with AK-47s.
At this point, Fairchild had spent $1.45 million in development expenses, and wished to divest itself of its small-arms business. Fairchild sold production rights for the AR-15 to Colt Firearms in December 1959, for only $75,000 cash and a 4.5% royalty on subsequent sales; Robert W. MacDonald of Cooper-MacDonald got a finder's fee of $250,000 and a 1% royalty for arranging the deal. In 1960, ArmaLite was reorganized, and Stoner left the company.
M16 adoption 
Curtis LeMay viewed a demonstration of the AR-15 in July 1960. In the summer of 1961, General LeMay had been promoted to the position of USAF Chief of Staff, and requested an order of 80,000 AR-15s for the U.S. Air Force. However, under the recommendation of General Maxwell D. Taylor, who advised the Commander in Chief that having two different calibers within the military system at the same time would be problematic, President Kennedy turned down the request. However, Advanced Research Projects Agency, which had been created in 1958 in response to the Soviet Sputnik program, embarked on project AGILE in the spring of 1961. AGILE's priority mission was to devise inventive fixes to the communist problem in South Vietnam. In October 1961, William Godel, a senior man at ARPA, sent 10 AR-15s to South Vietnam to let the allies test them. The reception was enthusiastic, and in 1962 another 1,000 AR-15s were sent to South Vietnam. Special Operations units and advisers working with the South Vietnamese troops filed battlefield reports lavishly praising the AR-15 and the stopping effectiveness of the 5.56 mm cartridge, and pressed for its adoption. However, what no one knew, except the men directly using the AR-15s in Vietnam, were the devastating kills made by the new rifle, photographs of which, showing enemy casualties made by the .223 (5.56 mm) bullet remained classified into the 1980s.
The damage caused by the .223 (5.56 mm) "varmint" bullet was observed and originally believed to be caused by "tumbling" due to the slow 1 in 14-inch (360 mm) rifling twist rate. However, this twist rate only made the bullet less stable in air. Any pointed lead core bullet will turn base over point ("tumble") after penetration in flesh, because the center of gravity is aft of the center of pressure. The large wounds observed by soldiers in Vietnam were actually caused by projectile fragmentation, which was created by a combination of the projectile's velocity and construction.
U.S. Secretary of Defense Robert McNamara now had two conflicting views: the ARPA report favoring the AR-15 and the Pentagon's position on the M14. Even President John F. Kennedy expressed concern, so McNamara ordered Secretary of the Army Cyrus Vance to test the M14, the AR-15 and the AK-47. The Army's test report stated that only the M14 was suitable for Army use, but Vance wondered about the impartiality of those conducting the tests. He ordered the Army Inspector General to investigate the testing methods used; the Insepctor General confirmed that the testers showed favor to the M14.
Secretary Robert McNamara ordered a halt to M14 production in January 1963, after receiving reports that M14 production was insufficient to meet the needs of the armed forces. Secretary McNamara had long been a proponent of weapons program consolidation among the armed services. At the time, the AR-15 was the only rifle that could fulfill a requirement of a "universal" infantry weapon for issue to all services. McNamara ordered the weapon be adopted unmodified, in its current configuration, for immediate issue to all services, despite receiving reports noting several deficiencies with the M16 as a service rifle, including the lack of a chrome-lined bore and chamber, the 5.56 mm projectile's instability under arctic conditions, and the fact that large quantities of 5.56 mm ammunition required for immediate service were not available. In addition, the Army insisted on the inclusion of a forward assist to help push the bolt into battery in the event that a cartridge failed to seat in the chamber through fouling or corrosion. Colt had argued the rifle was a self-cleaning design, requiring little or no maintenance. Colt, Eugene Stoner, and the U.S. Air Force believed that a forward assist needlessly complicated the rifle, adding about $4.50 to its procurement cost with no real benefit. As a result, the design was split into two variants: the Air Force's M16 without the forward assist, and, for the other service branches, the XM16E1 with the forward assist.
In November 1963, McNamara approved the U.S. Army's order of 85,000 XM16E1s for jungle warfare operations; and to appease General LeMay, the Air Force was granted an order for another 19,000 M16s. Meanwhile, the Army carried out another project, the Small Arms Weapons Systems, on general infantry firearm needs in the immediate future. They recommended the immediate adoption of the weapon. Later that year the Air Force officially accepted their first batch as the United States Rifle, Caliber 5.56 mm, M16.
The Army immediately began to issue the XM16E1 to infantry units. However, the rifle was initially delivered without adequate cleaning supplies or instructions and so, when the M16 reached Vietnam with U.S. troops in March 1965, reports of stoppages in combat began to surface. Often the gun suffered from a stoppage known as “failure to extract”, which meant that a spent cartridge case remained lodged in the chamber after a bullet flew out the muzzle. Although the M14 featured a chrome-lined barrel and chamber to resist corrosion in combat conditions, neither the bore nor the chamber of the M16/XM16E1 was chrome-lined. Several documented accounts of troops killed by enemy fire with inoperable rifles broken-down for cleaning eventually brought a Congressional investigation.
We left with 72 men in our platoon and came back with 19, Believe it or not, you know what killed most of us? Our own rifle. Practically every one of our dead was found with his [M16] torn down next to him where he had been trying to fix it.—Marine Corps Rifleman, Vietnam.
The root cause of the stoppages turned out to be a problem with the powder for the ammunition. In 1964, when the Army was informed that DuPont could not mass-produce the nitrocellulose-based powder to the specifications demanded by the M16, the Olin Mathieson Company provided a high-performance ball propellant of nitrocellulose and nitroglycerin. While the Olin WC 846 powder was capable of firing an M16 5.56 mm round at the desired 3,300 ft (1,000 m) per second, it had the unintended consequence of increasing the automatic rate of fire from 850 to 1,000 rounds per minute. This would leave behind dirty residue, making the M16 more likely to have a stoppage. The problem was resolved by fitting the M16 with a buffer system, slowing the rate of fire back down to 850 rounds per minute and outfitting all newly produced M16s with an anti corrosive chrome-plated chamber.
On February 28, 1967, the XM16E1 was standardized as the M16A1. Major revisions to the design followed. The rifle was given a chrome-lined chamber (and later, the entire bore) to eliminate corrosion and stuck cartridges, and the rifle's recoil mechanism was re-designed to accommodate Army-issued 5.56 mm ammunition. Rifle cleaning tools and powder solvents/lubricants were issued. Intensive training programs in weapons cleaning were instituted, and a comic book style manual was circulated among the troops to demonstrate proper maintenance. The reliability problems of the M16 diminished quickly, although the rifle's reputation continued to suffer.
According to a February 1968 Department of Defense report the M16 rifle achieved widespread acceptance by U.S. troops in Vietnam. Only 38 of 2,100 individuals queried wanted to replace the M16 with another weapon. Of those 38, 35 wanted the CAR-15 (a shorter version of the M16) instead.
NATO standards 
In March 1970, the U.S. stated that all NATO forces should eventually adopt the 5.56x45mm cartridge. This shift represented a change in the philosophy of the military's long-held position about caliber size. By the mid-1970s, other armies were also looking at M16-style weapons. A NATO standardization effort soon started, and tests of various rounds were carried out starting in 1977. The U.S. offered their original 5.56x45mm design, the M193, with no modifications, but there were concerns about its penetration in the face of the wider introduction of body armor. In the end, the Belgian 5.56x45mm SS109 round was chosen (STANAG 4172). This round was based on the U.S. cartridge, but included a new 62 grain bullet design with a small steel tip added to improve penetration. The U.S. Marine Corps was first to adopt the round with the M16A2, introduced in 1982. This was to become the standard U.S. military rifle. The NATO 5.56x45mm standard ammunition produced for U.S. forces is designated M855.
Shortly after, in October 1980, NATO accepted the 5.56x45mm NATO rifle cartridge. Draft Standardization Agreement 4179 (STANAG 4179) was proposed to allow the military services of member nations to easily share rifle ammunition and magazines during operations, at the individual soldier level, in the interest of easing logistical concerns. The magazine chosen to become the STANAG magazine was originally designed for the U.S. M16 rifle. Many NATO member nations, but not all, subsequently developed or purchased rifles with the ability to accept this type of magazine. However, the standard was never ratified and remains a 'Draft STANAG'
The NATO Accessory Rail STANAG 4694, or Picatinny rail STANAG 2324, or a "Tactical Rail" is a bracket used on M16 type rifles to provide a standardized mounting platform. The rail comprises a series of ridges with a T-shaped cross-section interspersed with flat "spacing slots". Scopes are mounted either by sliding them on from one end or the other; by means of a "rail-grabber" which is clamped to the rail with bolts, thumbscrews or levers; or onto the slots between the raised sections. The rail was originally for scopes. However, once established, the use of the system was expanded to other accessories, such as tactical lights, laser aiming modules, night vision devices, reflex sights, foregrips, bipods, and bayonets.
Currently, the M16 is in use by 15 NATO countries and more than 80 countries world wide.
Extended Magazines 
In addition to Colt's straight 20-round magazine, a proprietary fully curved 30-round magazine (like the AK-47) was also commercially available from 1964. However, it didn't work with all models of the rifle (or even some individual rifles) because the magazine well was designed to work with straight magazines and the weapon either would or wouldn't feed reliably when it used the extended magazine. When Special Forces personnel in Vietnam were trained on use of the XM177 Colt Commando assault-carbine in 1966, the few 30-round magazines used during the demonstrations were very popular. A request was made to have the XM177 "Commando" weapon ship with extended magazines if they could be made more reliable. Colt was too busy working on other technical issues of the AR-15 and XM177 to rework the extended magazines. Special Operations forces like the Rangers, Special Forces and SEALs tried to scrounge as many of the extended magazines as they could in-theater. The US Air Force, who had procured a small supply of the extended magazine for use by their Military Police and Air Commandos, had to secure them from being requisitioned by the other services.
An improved slightly curved 30-round magazine was finally developed by Colt in 1968 to meet a Special Operations requirement and began issue in 1969. It was mostly used by Special Operations forces, but began filtering through to regular units in the field. The improved extended magazine was adopted as Vietnam issue in 1970 when the M16A1 replaced the M14 as the military's standard service rifle. The improved extended 30-round magazine became military standard issue in 1975 and the original 20-round magazine became obsolete.
An experimental 50-round magazine was designed in 1969 by Naval Weapons Laboratory Dahlgren and was machined at Naval Ordnance Station Forest Park to meet a 1968 Navy SEALs requirement for an extended magazine. The prototype was made from welding several 20-round magazines together and used a longer and heavier magazine spring. This design had two problems. If a strong magazine spring was used, the magazine fed reliably but it was too strong to load it completely without a loading tool and the bottom rounds might jam. If a weaker magazine spring was used, it could be reloaded by hand but failed to feed smoothly, completely, or even at all. The Mark 1 prototype was made from welding a 20-round magazine to an improved extended 30-round magazine. It used two constant tension magazine springs that attached to the top of the magazine to pull the follower up rather than push it and used a low-friction plastic follower rather than an aluminum one - which allowed easy loading, steady feeding, and reduced jams. The later Mark 2 was similar but was fabricated like a regular magazine and had a sharper curve at the joint than the Mark 1 to allow more reliable feeding. The Mark 3 model incorporated improvements - one was a bolt hold-open device to indicate when the magazine was empty. SEAL Team 2 operators testing the Mark 3 model in the field found the magazine could only feed 45 rounds reliably (compared to 18 for the short magazine and 28 for the extended magazine) and the longer magazine made it difficult to fire from a prone position. The sustained firepower the magazine provided made up for these faults and a proposal was made in 1971 to issue at least one to every SEAL operative. As the Vietnam War began winding down, budget cuts forced the Navy to shelve the program in 1971 and no further magazines were made.
Additional firepower 
All current M16 type rifles are designed to fire STANAG (NATO standard) 22 mm rifle grenades from their integral flash hiders without the use of an adapter. These 22 mm grenade types range from powerful anti-tank rounds to simple finned tubes with a fragmentation hand grenade attached to the end. They come in the "standard" type which are propelled by a blank cartridge inserted into the chamber of the rifle. They also come in the "bullet trap" and "shoot through" types, as their names imply, they use live ammunition. The U.S. military does not generally use rifle grenades; however, they are used by other nations.
All current M16 type rifles can mount under-barrel 40 mm grenade-launchers, such as the M203 and M320. Both use the same 40 mm grenades as the older, stand-alone M79 grenade launcher. The M16 can also mount under-barrel 12 gauge shotguns such as KAC Masterkey or the M26 Modular Accessory Shotgun System.
Riot Control Launcher 
The M234 Riot Control Launcher is an M16 series rifle attachment firing a M755 blank round. The M234 mounts on the muzzle, bayonet lug and front sight post of the M16. It fires either the M734 64 mm Kinetic Riot Control or the M742 64 mm CSI Riot Control Ring Airfoil Projectiles. The latter produces a 4 to 5 foot Tear Gas cloud on impact. The main advantage to using Ring Airfoil Projectiles is that their design does not allow them be thrown back by rioters with any real effect. The M234 is no longer used by United States forces. It has been replaced by the M203 40mm grenade launcher and nonlethal ammunition.
The M16's receivers are made of 7075 aluminum alloy, its barrel, bolt, and bolt carrier of steel, and its handguards, pistol grip, and buttstock of plastics. Early models were especially lightweight at 6.5 pounds (2.9 kg) without magazine and sling. This was significantly less than older 7.62 mm "battle rifles" of the 1950s and 1960s. It also compares with the 6.5 pounds (2.9 kg) AKM without magazine. M16A2 and later variants (A3 & A4) weigh more (8.5 lb (3.9 kg) loaded) because of the adoption of a thicker barrel profile. The thicker barrel is more resistant to damage when handled roughly and is also slower to overheat during sustained fire. Unlike a traditional "bull" barrel that is thick its entire length, the M16A2's barrel is only thick forward of the handguards. The barrel profile under the handguards remained the same as the M16A1 for compatibility with the M203 grenade launcher. The rifle is the same length as the M16A2.
The rifling twist of early model M16 barrels had 4 grooves, right hand twist, 1 turn in 14 inches (1:355.6 mm) bore - as it was the same rifling used by the .222 Remington sporting round. This was shown to make the light .223 Remington bullet yaw in flight at long ranges and it was soon replaced. Later models had an improved rifling with 6 grooves, right hand twist, 1 turn in 12 inches (1:304.8 mm) for increased accuracy and was optimized for use with the standard US M193 cartridge. Current models are optimized for the heavier NATO SS109 bullet and have 6 grooves, right hand twist, 1 turn in 7 in (1:177.8 mm). Weapons designed to accept both the M193 or SS109 rounds (like civilian market clones) have a 6-groove, right hand twist, 1 turn in 9 inches (1:228.6 mm) bore.
The M16's most distinctive ergonomic feature is the carrying handle and rear sight assembly on top of the receiver. This is a by-product of the original design, where the carry handle served to protect the charging handle. As the line of sight is 2.5 in (63.5 mm) over the bore, the M16 has an inherent parallax problem. At closer ranges (typically inside 15–20 meters), the shooter must aim high in order to place shots where desired. The M16 has a 500mm (19.75 inches) sight radius. The M16 uses an L-type flip, aperture rear sight and it is adjustable with two setting, 0 to 300 meters and 300 to 400 meters. The front sight is a post adjustable for elevation in the field. The rear sight can be adjusted in the field for windage. The sights can be adjusted with a bullet tip and soldiers are trained to zero their own rifles. The sight picture is the same as the M14, M1 Garand, M1 Carbine and the M1917 Enfield. The M16 also has a "Low Light Level Sight System", which includes a front sight post with a small glass vial of (glow-in-the-dark) radioactive Tritium H3 and a larger aperture rear sight. The M16 can mount a scope on the carrying handle. With the advent of the M16A2, a new fully adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters and to allow windage adjustments without the need of a tool or cartridge. Modern versions of the M16 use a Picatinny rail, which allows the use of various scopes and sighting devices. The current United States Army and Air Force issue M4 Carbine comes with the M68 Close Combat Optic and Back-up Iron Sight. The United States Marine Corps uses the ACOG Rifle Combat Optic and the United States Navy uses EOTech Holographic Weapon Sight.
Another ergonomic feature is its straight-line recoil design, where the recoil spring is located in the stock directly behind the action. This serves the dual function of operating spring and recoil buffer. The stock being in line with the bore reduces muzzle rise, especially during automatic fire. Because recoil does not significantly shift the point of aim, faster follow-up shots are possible and user fatigue is reduced.
The M16 utilizes direct impingement gas operation; energy from high-pressure gas tapped from a non-adjustable port built into the front sight assembly actuates the moving parts in the weapon. Combustion gases travel via a gas tube above the barrel directly into a chamber in the bolt carrier behind the bolt itself, pushing the carrier away from the bolt. This reduces the number of moving parts by eliminating the need for a separate piston and cylinder and it provides better performance in rapid fire by keeping reciprocating masses on the same axis as the bore.
The primary criticism of direct impingement is that fouling and debris from expended gunpowder is blown directly into the breech. As the superheated combustion gas travels down the tube, it expands and cools. This cooling causes vaporized matter to condense as it cools, depositing a much greater volume of solids into the operating components of the action. The increased fouling can cause malfunctions if the rifle is not cleaned as frequently as it should be. The amount of sooting tends to vary with powder specification, caliber, and gas port design.
The M16 rifle fires the 5.56×45mm NATO cartridge and can produce massive wounding effects when the bullet impacts at high velocity and yaws in tissue leading to fragmentation and rapid transfer of energy. This produces wounds that were so devastating that the International Committee of the Red Cross (ICRC) and many countries considered the M16 to be an inhumane weapon.
Pre-Production ArmaLite AR-15 
The weapon that eventually became the M16 series only had a vague resemblance to its offspring. The rifle was basically a scaled down AR-10 with an ambidextrous charging handle located within the carrying handle, a narrower front sight "A" frame, and no flash suppressor.
AR-15 (Colt Models 601 & 602) 
Colt's first two models produced after the acquisition of the rifle from ArmaLite were the 601 and 602, and these rifles were in many ways clones of the original ArmaLite rifle (in fact, these rifles were often found stamped Colt ArmaLite AR-15, Property of the U.S. Government caliber .223, with no reference to them being M16s). The 601 and 602 are easily identified by their flat lower receivers without raised surfaces around the magazine well and occasionally green or brown furniture. The 601 was adopted first of any of the rifles by the USAF, and was quickly supplemented with the XM16 (Colt Model 602) and later the M16 (Colt Model 604) as improvements were made. There was also a limited purchase of 602s, and a number of both of these rifles found their way to a number of Special Operations units then operating in South East Asia, most notably the U.S. Navy SEALs. The only major difference between the 601 and 602 is the switch from the original 1:14-inch rifling twist to the more common 1:12-inch twist. These weapons were equipped with a triangular charging handle and a bolt hold open device that lacked a raised lower engagement surface. The bolt hold open device had a slanted and serrated surface that had to be engaged with a bare thumb, index finger, or thumb nail because of the lack of this surface.
The United States Air Force continued to use the AR-15 marked rifles in various configurations into the 1990s.
Variant originally adopted by the U.S. Air Force. This was the first M16 adopted operationally. This variant had triangular handguards, butt stocks without a compartment for the storage of a cleaning kit, a three-pronged flash suppressor, and no forward assist. Bolt carriers were originally chrome plated and slick-sided, lacking forward assist notches. Later, the chrome plated carriers were dropped in favor of Army issued notched and parkerized carriers though the interior portion of the bolt carrier is still chrome-lined. The Air Force continued to operate these weapons until around 2001, at which time the Air Force converted all of its M16s to the M16A2 configuration.
XM16E1 and M16A1 (Colt Model 603) 
The U.S. Army XM16E1 was essentially the same weapon as the M16 with the addition of a forward assist and corresponding notches in the bolt carrier. The M16A1 was the finalized production model in 1967.
To address issues raised by the XM16E1's testing cycle, a closed, bird-cage flash suppressor replaced the XM16E1's three-pronged flash suppressor which caught on twigs and leaves. Various other changes were made after numerous problems in the field. Cleaning kits were developed and issued while barrels with chrome-plated chambers and later fully lined bores were introduced. The front sight was now a square post with 4 detent positions, adjustable for vertical zeroing by using a cartridge, nail or special tool.
With these and other changes, the malfunction rate slowly declined and new soldiers were generally unfamiliar with early problems. A rib was built into the side of the receiver on the XM16E1 to help prevent accidentally pressing the magazine release button while closing the ejection port cover. This rib was later extended on production M16A1s to help in preventing the magazine release from inadvertently being pressed. The hole in the bolt that accepts the cam pin was crimped inward on one side, in such a way that the cam pin may not be inserted with the bolt installed backwards, which would cause failures to eject until corrected. The M16A1 is no longer in service with the United States, but is still standard issue in many world armies.
The development of the M16A2 rifle was originally requested by the United States Marine Corps as a result of the USMC's combat experience in Vietnam with the XM16E1 and M16A1. The Marines were the first branch of the U.S. Armed Forces to adopt the M16A2 in the early/mid-1980s, with the United States Army following suit in the late 1980s. Modifications to the M16A2 were extensive. In addition to the new rifling, the barrel was made with a greater thickness in front of the front sight post, to resist bending in the field and to allow a longer period of sustained fire without overheating. The rest of the barrel was maintained at the original thickness to enable the M203 grenade launcher to be attached. A new adjustable rear sight was added, allowing the rear sight to be dialed in for specific range settings between 300 and 800 meters to take full advantage of the ballistic characteristics of the new SS109 rounds and to allow windage adjustments without the need of a tool or cartridge. The flash suppressor was again modified, this time to be closed on the bottom so it would not kick up dirt or snow when being fired from the prone position, and acting as a recoil compensator. The front grip was modified from the original triangular shape to a round one, which better fitted smaller hands and could be fitted to older models of the M16. The new handguards were also symmetrical so that armories need not separate left and right spares. The handguard retention ring was tapered to make it easier to install and uninstall the handguards. A notch for the middle finger was added to the pistol grip, as well as more texture to enhance the grip. The buttstock was lengthened by 5⁄8 in (15.9 mm). The new buttstock became ten times stronger than the original due to advances in polymer technology since the early 1960s. Original M16 stocks were made from fiberglass-impregnated resin; the newer stocks were engineered from DuPont Zytel glass-filled thermoset polymers. The new stock included a fully textured polymer buttplate for better grip on the shoulder, and retained a panel for accessing a small compartment inside the stock, often used for storing a basic cleaning kit. The heavier bullet reduces muzzle velocity from 3,200 feet per second (980 m/s), to about 3,050 feet per second (930 m/s). The A2 also uses a faster twist rifling to allow the use of a trajectory-matched tracer round. A spent case deflector was incorporated into the upper receiver immediately behind the ejection port to prevent cases from striking left-handed users.
The action was also modified, replacing the fully automatic setting with a three-round burst setting. When using a fully automatic weapon, inexperienced troops often hold down the trigger and "spray" when under fire. The U.S. Army concluded that three-shot groups provide an optimum combination of ammunition conservation, accuracy and firepower. The USMC has retired the M16A2 in favor of the newer M16A4. However, many M16A2s remain in US Army, Air Force, Navy and Coast Guard service.
The M16A3 is a fully automatic variant of the M16A2 adopted in small numbers around the time of the introduction of the M16A2, primarily by the U.S. Navy for use by SEAL, Seabee, and Security units. It features the M16A1 trigger group providing "safe", "semi-automatic", and "fully automatic" modes.
The M16A4 is the fourth generation of the M16 series. It is equipped with a removable carrying handle and a full length quad Picatinny rail for mounting optics and other ancillary devices. The FN M16A4, using safe/semi/burst selective fire, is now the standard issue for all U.S. Marine Corps and is the current issue to Marine Corps' recruits in both MCRD San Diego and MCRD Parris Island.
Military issue rifles are also equipped with a Knight's Armament Company M5 RAS hand guard, allowing vertical grips, lasers, tactical lights, and other accessories to be attached, coining the designation M16A4 MWS (or Modular Weapon System) in U.S. Army field manuals.
Colt also produces M16A4 models for international purchases, with specifics selective fire:
- R0901 / NSN 1005-01-383-2872 (Safe/Semi/Auto)
- R0905 (Safe/Semi/Burst)
The Marine Corps is considering a Product Improvement Program (PIP) for their M16A4 MWS rifles. Potential features include a modular and adjustable stock, ambidextrous fire selector, heavier barrel, improved trigger, free-floating rail system, and an adjustable gas block and gas regulator to give all rifles a suppression capability.
|Colt model no.||Military designation||Barrel Length||Barrel||Handguard type||Buttstock type||Pistol grip type||Lower receiver type||Upper receiver type||Rear sight type||Front sight type||Muzzle device||Forward assist?||Case deflector?||Bayonet lug?||Trigger pack|
|601||AR-15||20 in (508 mm)||A1 profile (1:14 twist)||Green or brown full-length triangular||Green or brown fixed A1||A1||A1||A1||A1||A1||Duckbill flash suppressor||No||No||Yes||Safe-Semi-Auto|
|602||AR-15 or XM16||20 in (508 mm)||A1 profile (1:12 twist)||Full-length triangular||Fixed A1||A1||A1||A1||A1||A1||Duckbill or three-prong flash suppressor||No||No||Yes||Safe-Semi-Auto|
|603||XM16E1||20 in (508 mm)||A1 profile (1:12 twist)||Full-length triangular||Fixed A1||A1||A1||A1||A1||A1||Three-prong or M16A1 birdcage flash suppressor||Yes||No||Yes||Safe-Semi-Auto|
|603||M16A1||20 in (508 mm)||A1 profile (1:12 twist)||Full-length triangular||Fixed A1||A1||A1||A1||A1||A1||Three-prong or birdcage flash suppressor||Yes||No||Yes||Safe-Semi-Auto|
|604||M16||20 in (508 mm)||A1 profile (1:12 twist)||Full-length triangular||Fixed A1||A1||A1||A1||A1||A1||Three-prong or M16A1-style birdcage flash suppressor||No||No||Yes||Safe-Semi-Auto|
|645||M16A1E1/PIP||20 in (508 mm)||A2 profile (1:7 twist)||Full-length ribbed||Fixed A2||A1||A1 or A2||A1 or A2||A1 or A2||A2||M16A1 or M16A2-style birdcage flash suppressor||Yes||Yes or No||Yes||Safe-Semi-Auto or Safe-Semi-Burst|
|645||M16A2||20 in (508 mm)||A2 profile (1:7 twist)||Full-length ribbed||Fixed A2||A2||A2||A2||A2||A2||M16A2-style birdcage flash suppressor||Yes||Yes||Yes||Safe-Semi-Burst|
|645E||M16A2E1||20 in (508 mm)||A2 profile (1:7 twist)||Full-length ribbed||Fixed A2||A2||A2||Flattop with Colt Rail||Flip-up||Folding||M16A2-style birdcage flash suppressor||Yes||Yes||Yes||Safe-Semi-Burst|
|N/A||M16A2E2||20 in (508 mm)||A2 profile (1:7 twist)||Full-length semi-beavertail w/ HEL guide||Retractable ACR||ACR||A2||Flattop with Colt rail||None||A2||ACR muzzle brake||Yes||Yes||Yes||Safe-Semi-Burst|
|646||M16A2E3/M16A3||20 in (508 mm)||A2 profile (1:7 twist)||Full-length ribbed||Fixed A2||A2||A2||A2||A2||A2||M16A2-style birdcage flash suppressor||Yes||Yes||Yes||Safe-Semi-Auto|
|655||M16A1 Special High Profile||20 in (508 mm)||HBAR profile (1:12 twist)||Full-length triangular||Fixed A1||A1||A1||A1||A1||A1||M16A1-style birdcage flash suppressor||Yes||No||Yes||Safe-Semi-Auto|
|656||M16A1 Special Low Profile||20 in (508 mm)||HBAR profile (1:12 twist)||Full-length triangular||Fixed A1||A1||A1||A1 with modified Weaver base||Low Profile A1||Hooded A1||M16A1-style birdcage flash suppressor||Yes||No||Yes||Safe-Semi-Auto|
|945||M16A2E4/M16A4||20 in (508 mm)||A2 profile (1:7 twist)||Full-length ribbed or KAC M5 RAS||Fixed A2||A2||A2||Flattop with MIL-STD-1913 rail||None||A4||M16A2-style birdcage flash suppressor||Yes||Yes||Yes||Safe-Semi-Burst|
|Colt model no.||Military designation||Barrel Length||Barrel||Handguard type||Buttstock type||Pistol grip type||Lower receiver type||Upper receiver type||Rear sight type||Front sight type||Muzzle device||Forward assist?||Case deflector?||Bayonet lug?||Trigger pack|
Colt Model 655 and 656 "Sniper" variants 
With the expanding Vietnam War, Colt developed two rifles of the M16 pattern for evaluation as possible light sniper or designated marksman rifles. The Colt Model 655 M16A1 Special High Profile was essentially a standard A1 rifle with a heavier barrel and a scope mount that attached to the rifle's carry handle. The Colt Model 656 M16A1 Special Low Profile had a special upper receiver with no carrying handle. Instead, it had a low-profile iron sight adjustable for windage and a Weaver base for mounting a scope, a precursor to the Colt and Picatinny rails. It also had a hooded front iron sight in addition to the heavy barrel. Both rifles came standard with either a Leatherwood/Realist scope 3–9× Adjustable Ranging Telescope. Some of them were fitted with a Sionics noise and flash suppressor. Neither of these rifles were ever standardized.
In Vietnam, some soldiers were issued a carbine version of the M16 called the XM177. The XM177 had a shorter 10 in (254 mm) barrel and a telescoping stock, which made it substantially more compact. It also possessed a combination flash hider/sound moderator to reduce problems with muzzle flash and loud report. The USAF's GAU-5/A (XM177) and the U.S. Army's XM177E1 variants differed over the latter’s inclusion of a forward assist, although some GAU-5s do have the forward assist. The final USAF GAU-5A/A and U.S. Army XM177E2 had an 11.5 in (292 mm) barrel with a longer flash/sound suppressor. The lengthening of the barrel was to support the attachment of Colt's own XM148 40 mm grenade launcher. These versions were also known as the Colt Commando model commonly referenced and marketed as the CAR-15. The variants were issued in limited numbers to special forces, helicopter crews, Air Force pilots, Air Force Security Police Military Working Dog (MWD) handlers, officers, radio operators, artillerymen, and troops other than front line riflemen. Some USAF GAU-5A/As were later equipped with even longer 14.5-inch (370 mm) 1/12 rifled barrels as the two shorter versions were worn out. The 14.5-inch (370 mm) barrel allowed the use of MILES gear and for bayonets to be used with the sub-machine guns (as the USAF described them). By 1989, the USAF started to replace the earlier barrels with 1/7 rifled models for use with the M855 round. The weapons were given the redesignation of GUU-5/P.
Colt Model 733 
Colt also returned to the original "Commando" idea, with its Model 733, essentially a modernized XM177E2 with many of the features introduced on the M16A2.
M231 Firing Port Weapon (FPW) 
M231 Firing Port Weapon (FPW) is an adapted version of the M16 assault rifle for firing from ports on the M2 Bradley. The infantry's normal M16s are too long for use in a "buttoned up" fighting vehicle, so the FPW was developed to provide a suitable weapon for this role. Designed by the Rock Island Arsenal, the M231 FPW remains in service, although all but the rear two firing ports on the Bradley have been removed. The M231 FPW fires from the open bolt and is only configured for fully automatic fire. The open bolt configuration gives the M231 a much higher cyclic rate of fire than the closed bolt operation of the M16A1. Official doctrine discourages deploying M231 outside of the firing port role. The weapon jams easily and is known to break the bolt without warning.
Mk 4 Mod 0 
The Mk 4 Mod 0 was a variant of the M16A1 produced for the U.S. Navy SEALs during the Vietnam War and adopted in April 1970. It differed from the basic M16A1 primarily in being optimized for maritime operations and coming equipped with a sound suppressor. Most of the operating parts of the rifle were coated in Kal-Guard, a hole of 0.25 inches (6.4 mm) was drilled through the stock and buffer tube for drainage, and an O-ring was added to the end of the buffer assembly. The weapon could reportedly be carried to the depth of 200 feet (60 m) in water without damage. The initial Mk 2 Mod 0 Blast Suppressor was based on the U.S. Army's Human Engineering Lab's (HEL) M4 noise suppressor. The HEL M4 vented gas directly from the action, requiring a modified bolt carrier. A gas deflector was added to the charging handle to prevent gas from contacting the user. Thus, the HEL M4 suppressor was permanently mounted though it allowed normal semi-automatic and automatic operation. If the HEL M4 suppressor were removed, the weapon would have to be manually loaded after each single shot. On the other hand, the Mk 2 Mod 0 blast suppressor was considered an integral part of the Mk 4 Mod 0 rifle, but it would function normally if the suppressor were removed. The Mk 2 Mod 0 blast suppressor also drained water much more quickly and did not require any modification to the bolt carrier or to the charging handle. In the late 1970s, the Mk 2 Mod 0 blast suppressor was replaced by the Mk 2 blast suppressor made by Knight's Armament Company (KAC). The KAC suppressor can be fully submerged and water will drain out in less than eight seconds. It will operate without degradation even if the rifle is fired at the maximum rate of fire. The U.S. Army replaced the HEL M4 with the much simpler Studies in Operational Negation of Insurgency and Counter-Subversion (SIONICS) MAW-A1 noise and flash suppressor.
Mark 12 
Developed to increase the effective range of soldiers in the designated marksman role, the US Navy developed the Mark 12 Special Purpose Rifle (SPR). Configurations in service vary, but the core of the Mark 12 SPR is an 18" heavy barrel with muzzle brake and free float tube. This tube relieves pressure on the barrel caused by standard handguards and greatly increases the potential accuracy of the system. Also common are higher magnification optics ranging from the 6× power Trijicon ACOG to the Leupold Mark 4 Tactical rifle scopes. Firing Mark 262 Mod 0 ammunition with a 77gr Open tip Match bullet, the system has an official effective range of 600+ meters. However published reports of confirmed kills beyond 800 m from Iraq and Afghanistan are not uncommon.
M4 carbine 
The M4 carbine was developed from various outgrowths of these designs, including a number of 14.5-inch (368 mm)-barreled A1 style carbines. The XM4 (Colt Model 727) started its trials in the mid-1980s, with a barrel of 14.5 inches (370 mm). Officially adopted as a replacement for the M3 "Grease Gun" (and the Beretta M9 and M16A2 for select troops) in 1994, it was used with great success in the Balkans and in more recent conflicts, including the Afghanistan and Iraq theaters. The M4 carbine has a three-round burst firing mode, while the M4A1 carbine has a fully automatic firing mode. Both have a Picatinny rail on the upper receiver, allowing the carry handle/rear sight assembly to be replaced with other sighting devices.
International derivatives 
C7 and C8 
The Diemaco C7 and C8 are updated variants of the M16 developed and used by the Canadian Forces and are now manufactured by Colt Canada. The C7 is a further development of the experimental M16A1E1. Like earlier M16s, it can be fired in either single shot or automatic mode, instead of the burst function selected for the M16A2. The C7 also features the structural strengthening, improved handguards, and longer stock developed for the M16A2. Diemaco changed the trapdoor in the buttstock to make it easier to access and a spacer of 0.5 inches (13 mm) is available to adjust stock length to user preference. The most easily noticeable external difference between American M16A2s and Diemaco C7s is the retention of the A1 style rear sights. Not easily apparent is Diemaco's use of hammer-forged barrels. The Canadians originally desired to use a heavy barrel profile instead.
The C7 has been developed to the C7A1, with a Weaver rail on the upper receiver for a C79 optical sight, and to the C7A2, with different furniture and internal improvements. The Diemaco produced Weaver rail on the original C7A1 variants does not meet the M1913 'Picatinny' standard, leading to some problems with mounting commercial sights. This is easily remedied with minor modification to the upper receiver or the sight itself. Since Diemaco's acquisition by Colt to form Colt Canada, all Canadian produced flattop upper receivers are machined to the M1913 standard.
The C8 is the carbine version of the C7. The C7 and C8 are also used by Hærens Jegerkommando, Marinejegerkommandoen and FSK (Norway), Military of Denmark (all branches), the Royal Netherlands Army and Netherlands Marine Corps as its main infantry weapon. Following trials, variants became the weapon of choice of the British SAS.
- The Chinese Norinco CQ is an unlicensed derivative of the M16A1 made specifically for export, with the most obvious external differences being in its handguard and revolver-style pistol grip.
- Terab rifle is the Norinco CQ manufactured by the MIC (Military Industry Corporation) of Sudan
- ARMADA rifle and TRAILBLAZER carbine are the CQ and the CQ Type A are manufactured by S.A.M. – Shooter's Arms Manufacturing, a.k.a. Shooter's Arms Guns & Ammo Corporation, headquartered in Metro Cebu, Republic of the Philippines.
- The S-5.56 rifle, a clone of the Type CQ manufactured by the Defense Industries Organization of Iran The rifle itself is offered in two variants: the S-5.56 A1 with a 19.9-inch barrel and 1:12 pitch rifling (1 turn in 305mm), optimized for the use of the M193 Ball cartridge; and the S-5.56 A3 with a 20-inch barrel and a 1:7 pitch rifling (1 turn in 177, 8mm), optimized for the use of the SS109 cartridge.
- The KH-2002 is an Iranian bullpup conversion of the locally produced S-5.56 rifle. Iran intends to replace the standard issue weapon of its armed forces with this rifle.
- The MSSR rifle developed as an effective, low cost sniper rifle by the Philippine Marine Corps Scout Snipers. The Special Operations Assault Rifle (SOAR) assault carbine was developed by Ferfrans based on the M16 rifle. It is used by the Special Action Force.
- Taiwan uses piston-driven M16-based weapons as their standard rifle. These include the T65, T86 and T91 assault rifles.
Production and users 
The M16 is the most commonly manufactured 5.56x45 mm rifle in the world. Currently, the M16 is in use by 15 NATO countries and more than 80 countries world wide. Together, numerous companies in the United States, Canada, and China have produced more than 8,000,000 rifles of all variants. Approximately 90% are still in operation. The M16 replaced the M14 and M1 carbine as standard infantry rifles of the U.S. armed forces. The M14 continues to see limited service, mostly in sniper, designated marksman, and ceremonial roles.
- Afghanistan C7 briefly used by Afghan National Army
- Argentina: Special Forces used M16A1 in the Falklands (Malvinas) War and they currently use the M16A2.
- Australia M16A1 introduced during the Vietnam War and replaced by the F88 Austeyr in 1989.
- Bangladesh: Used by the military, special forces and counter terrorism units.
- Cambodia M16A1 is used.
- Canada: C7 and C8 variants made by Colt Canada is used by the Canadian Forces.
- Colombia
- Costa Rica
- Democratic Republic of the Congo
- Denmark: C7 and C8 variants made by Colt Canada are used by all branches of the Danish Defence.
- Dominican Republic
- East Timor
- El Salvador M16A1/A2 is used.
- Greece M16A2/M4 is used by the Special forces of the Hellenic Army ISAF Forces in Afghanistan and Hellenic Navy
- Guatemala M16A1 is used.
- Iraq: Used by Iraqi Army.
- Israel - Being replaced by IMI Tavor.
- Republic of Korea: During Vietnam War, United States provided 27,000 M16 rifles to Republic of Korea Armed Forces in Vietnam. Also, 600,000 M16A1s (Colt Model 603K) were manufactured under license by Daewoo Precision Industries. The delivery started in 1974 and ended in 1985.
- Lithuania: Lithuanian Armed Forces.
- Monaco: Compagnie des Carabiniers du Prince.
- New Zealand M16 was used and was replaced in 1988 by Steyr AUG
- Netherlands: C7 and C8 variants are used by the Royal Netherlands Army.
- Pakistan: Special Service Group (SSG) of the Pakistan Army.
- Panama M16A1 is used.
- Philippines: Manufactured under license by Elisco Tool and Manufacturing.
- Saudi Arabia : M16 used by Saudi Arabian Army.
- Singapore: Local variant of the M16A2 (M16S1) manufactured under license by ST Kinetics.
- South Africa
- Sri Lanka
- South Vietnam: 6,000 M16 and 938,000 M16A1, 1966–1975
- Sweden A small number of M16A2's are used by the Swedish Armed Forces for familiarization training (as well as a similar number of AKM's), but they are not issued to combat units. C8 used by Special Forces
- Tunisia M16A2/A4 variants are used.
- Turkey M16A1/A2/A4 variants are used.
- United Arab Emirates
- United Kingdom: C7/C8 (L119A1) variant are used by the Royal Military Police, the Pathfinder Platoon of the Parachute Regiment and the United Kingdom Special Forces
- United States
Future replacement 
Previous attempts by the U.S. Military to replace the M16 were unsuccessful or only supplemented it. The M4 carbine will eventually replace the M16 rifle in most combat units in the U.S. Army.
Throughout the 1970s, the Army experimented with various materials to replace brass in cartridge casings. Brass has a number of qualities that make it almost ideal for a cartridge, including low friction against steel, making it easier to extract, and the ease with which casings can be manufactured. However, brass is also dense and expensive, so replacing it could lower both the cost and weight of the ammunition.
Aluminum and steel were popular materials for complete rounds, and AAI successfully developed a plastic blank. Completely caseless ammunition was also studied on several occasions, notably the German 4.7 mm designs, and this concept is now being continued with the Lightweight Small Arms Technologies Program.
Later in the 1980s, the Advanced Combat Rifle program was run to find a replacement for the M16. Colt entered a modified M16A2 known as the Colt ACR, which used duplex rounds, a system that lowered recoil by 40% to improve repeating shots, and added a 3.5x scope. This weapon, designated M16A2E2, also featured a "guide" of sorts as part of a special handguard developed by the U.S. Army Human Engineering Laboratory designed to assist in snap-shooting, and a carbine style stock very similar to the recent stock developed by the Naval Surface Warfare Center Crane Division.
The Steyr ACR used new flechette ammunition that was nominally called 5.56 mm, with a very high 4,750 ft/s (1,448 m/s) muzzle velocity. Other variants experimented with caseless ammunition technologies as well.
In the 1980s, the M249 was issued to infantry units, replacing some M60s and some M16A1s at the squad level. In the 1990s the M4 carbine took over the operational role of the M3 submachine gun, some M9s, and many M16A2s. The U.S. Air Force mostly uses M4 and GUU-5/P carbines for security squadrons and M16A2s for non-security personnel. The USMC has retired the M16A2 in favor of the M16A4 rifle which share almost all the same parts of the A2 but has an optics rail for USMC RCO ACOG scopes. U.S. Navy has retained the M16A2 and M16A3 for its units that use rifles like the U.S. Navy Seabees. The U.S. Army has largely relegated the M16A2 to non-combat roles, choosing instead the M4 and M4A1. Further, the M16 never entirely replaced the M14 in all roles, which continues to be used in a number of niche applications throughout the Armed Forces, especially with the U.S. Navy.
Replacement designs 
Replacement of the M16 family has been proposed at various points, and its longevity is in part due to a series of failures in projects meant to replace it, driven largely by the requirement for a significant improvement. Immediately after the introduction of the M16, the Marine Corps sought to adopt the Stoner 63. Although they found it superior in most ways, it was still at an early stage of development; the Marines chose the technically inferior but mature M16. The Advanced Combat Rifle program in the 1980s produced weapons that were superior in some ways, but none improved upon the M16 series enough to replace it. It was also potentially going to be replaced by the SABR, from the OICW project. The weapon system originally planned by the OICW project was put on hold around the turn of century, in favor of a simpler new 5.56 mm rifle project that offered less far-reaching improvements. The resulting XM8 rifle was also intended as a potential replacement for the M16 family. However, this program too ran into problems around 2004–05, and was put on hold in favor of an open competition for what became known as the OICW Increment 1. (Increment 2 is the standalone XM25 Individual Airburst Weapon System, and Increment 3 is the XM29 OICW, a weapon that combined the earlier two increments.) This competition was subsequently put on hold in the summer of 2005 to take into account input from other services; then the competition was cancelled in October 2005.
A partial replacement for the M16 rifle is the SOF Combat Assault Rifle (SCAR), designated Mk 16. The 5.56x45 mm Mk 16 emerged as the winner of a USSOCOM competition to find a new rifle for Special Operations Forces in 2003 and entered operational service in 2009. Most of the SCAR's basic controls (pistol grip, magazine release, selector lever, and bolt release) share the same location and function as on the M16 and M4 they are supplementing.
Since the cancellation of the XM8 program, and into 2008, the LSAT rifle program has been pursued for the next generation of U.S. small arms. The program has already produced practical results and is currently projected to provide the next U.S. military rifle. Development of the LSAT rifle began in 2008.
Several companies have been working on and creating potential candidates for the U.S. Army's next primary weapon. The Heckler & Koch HK416 has been in use by Delta Force since 2004. The H&K's design replaces the direct impingement gas system with a newer piston design. The HK416 is offered as a conversion kit that can retrofit current M4 carbines. The HK416 recently won a testing competition for the US Marine Corps Infantry Automatic Rifle program and entered final testing as the M27 in summer 2010. The M27 entered service in 2011, but as a replacement for the M249 SAW, not the M16. The HK416 is also one of several M4 designs on the front competitors of the Army's Individual Carbine competition. The reason to build a new M4 is to produce a new more reliable carbine for US troops in a few years. However, in November 2011, leaders of the US Marine Corps announced they will stay with current M4 Carbines and M16A4s and not adopt an IC competition winner. The USMC is considering upgrading the M16A4. However, the Marines are watching the Individual Carbine competition and has said whether or not they will join is yet to be seen. The Army is considering cancelling the Individual Carbine competition.
Gas Piston 
Complicating the Army search for higher reliability in the M4 is a number of observations of M4 gas piston alternatives that suffer unintended design problems. The first is that many of the gas piston modifications for the M4 isolate the piston so that piston jams or related malfunction require the entire weapon be disassembled, such disassembly cannot be performed by the end user and requires a qualified armorer to perform out of field, whereas any malfunction with the direct-impingement system can be fixed by the end user in field. The second is that gas piston alternatives use an off-axis operation of the piston that can introduce carrier tilt, whereby the bolt carrier fails to enter the buffer tube at a straight angle resulting in part wearing. The third is that the use of a sound suppressor results in hot gases entering the chamber, regardless of a direct-gas impingement or gas piston design choice.
See also 
- Adaptive Combat Rifle
- AR-15 variants – list of all AR-15 and M16 variants
- Colt 9mm SMG
- Comparison of the AK-47 and M16
- Daewoo K2, Republic of Korea Armed Forces (South Korea) assault rifle
- List of individual weapons of the U.S. armed forces
- M203 40 mm grenade launcher
- MSBS Radon
- Norinco CQ, M16 clone developed by China
- Robinson Arms XCR
- T65 assault rifle, AR-15 variant developed by ROC Army
- Winchester LMR
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