The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of assault rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-round magazine.
In 1964, the XM16E1 entered US military service as the M16 and in the following year was deployed for jungle warfare operations during the Vietnam War.[13] In 1969, the M16A1 replaced the M14 rifle to become the US military's standard service rifle.[14] The M16A1 incorporated numerous modifications including a bolt-assist, chrome-plated bore, protective reinforcement around the magazine release, and revised flash hider.[13]
In 1983, the US Marine Corps adopted the M16A2 rifle, and the US Army adopted it in 1986. The M16A2 fires the improved 5.56×45mm (M855/SS109) cartridge and has a newer adjustable rear sight, case deflector, heavy barrel, improved handguard, pistol grip, and buttstock, as well as a semi-auto and three-round burst fire selector.[15] Adopted in July 1997, the M16A4 is the fourth generation of the M16 series. It is equipped with a removable carrying handle and quad Picatinny rail for mounting optics and other ancillary devices.[16]
The M16 has also been widely adopted by other armed forces around the world. Total worldwide production of M16s is approximately 8 million, making it the most-produced firearm of its 5.56 mm caliber.[17][1] The US military has largely replaced the M16 in frontline combat units with a shorter and lighter version, the M4 carbine.[18][19] In April 2022, the U.S. Army selected the SIG MCX SPEAR as the winner of the Next Generation Squad Weapon Program to replace the M16/M4. The new rifle is designated XM7.[20]
In 1928, a U.S. Army 'Caliber Board' conducted firing tests at Aberdeen Proving Ground and recommended transitioning to smaller caliber rounds, mentioning, in particular .27 in (6.86 mm) caliber. Largely in deference to tradition, this recommendation was ignored and the Army referred to the .30 in (7.62 mm) caliber as "full-sized" for the next 35 years.[21] After World War II, the United States military started looking for a single automatic rifle to replace the M1 Garand, M1/M2 carbines, M1918 Browning automatic rifle, M3 "Grease Gun" and Thompson submachine gun.[22] However, early experiments with select-fire versions of the M1 Garand proved disappointing.[23] During the Korean War, the select-fire M2 carbine largely replaced the submachine gun in US service[24] and became the most widely used carbine variant.[25] However, combat experience suggested that the .30 carbine round was underpowered.[26] American weapons designers concluded that an intermediate round was necessary, and recommended a small-caliber, high-velocity cartridge.[27]
However, senior American commanders, having faced fanatical enemies and experienced major logistical problems during World War II and the Korean War,[28][29] insisted that a single, powerful .30 caliber cartridge be developed, that could not only be used by the new automatic rifle but by the new general-purpose machine gun (GPMG) in concurrent development.[30] This culminated in the development of the 7.62×51 mm NATO cartridge.[31]
The U.S. Army then began testing several rifles to replace the obsolete M1. Springfield Armory's T44E4 and heavier T44E5 were essentially updated versions of the M1 chambered for the new 7.62 mm round, while Fabrique Nationale submitted their FN FAL as the T48. ArmaLite entered the competition late, hurriedly submitting several AR-10 prototype rifles in the fall of 1956 to the U.S. Army's Springfield Armory for testing.[32] The AR-10 featured an innovative straight-line barrel/stock design, forged aluminum alloy receivers, and with phenolic composite stocks.[33] It had rugged elevated sights, an oversized aluminum[note 1] flash suppressor and recoil compensator, and an adjustable gas system.[35] The final prototype featured an upper and lower receiver with the now-familiar hinge and takedown pins, and the charging handle was on top of the receiver placed inside of the carry handle.[32] For a 7.62 mm NATO rifle, the AR-10 was incredibly lightweight at only 6.85 lb (3.11 kg) empty.[36] Initial comments by Springfield Armory test staff were favorable, and some testers commented that the AR-10 was the best lightweight automatic rifle ever tested by the Armory.[37] In the end, the U.S. Army chose the T44, now named the M14 rifle,[31] which was an improved M1 Garand with a 20-round magazine and automatic fire capability.[38][note 2] The U.S. also adopted the M60 general-purpose machine gun (GPMG).[31] Its NATO partners adopted the FN FAL and HK G3 rifles, as well as the FN MAG and Rheinmetall MG3 GPMGs.
The first confrontations between the AK-47 and the M14 came in the early part of the Vietnam War. Battlefield reports indicated that the M14 was uncontrollable in full-auto and that soldiers could not carry enough ammunition to maintain fire superiority over the AK-47.[40] And, while the M2 carbine offered a high rate of fire, it was under-powered and ultimately outclassed by the AK-47.[41] A replacement was needed: a medium between the traditional preference for high-powered rifles such as the M14, and the lightweight firepower of the M2 carbine.[42]
As a result, the Army was forced to reconsider a 1957 request by General Willard G. Wyman, commander of the U.S. Continental Army Command (CONARC) to develop a .223-inch caliber (5.56 mm) select-fire rifle weighing 6 lb (2.7 kg) when loaded with a 20-round magazine.[43] The 5.56 mm round had to penetrate a standard U.S. helmet at 500 yards (460 meters) and retain a velocity over the speed of sound while matching or exceeding the wounding ability of the .30 carbine cartridge.[44]
This request ultimately resulted in the development of a scaled-down version of the Armalite AR-10, named the ArmaLite AR-15.[45] The AR-15 was first revealed by Eugene Stoner at Fort Benning in May 1957.[46] The AR-15 used .22-caliber bullets, which destabilized when they hit a human body, as opposed to the .30 round, which typically passed through in a straight line. The smaller caliber meant that it could be controlled in autofire due to the reduced bolt thrust and free recoil impulse. Being almost one-third the weight of the .30 meant that the soldier could sustain fire for longer with the same load. Due to design innovations, the AR-15 could fire 600 to 700 rounds a minute with an extremely low jamming rate. Parts were stamped out, not hand-machined, so they could be mass-produced, and the stock was plastic to reduce weight.[21]
In 1958, the Army's Combat Developments Experimentation Command ran experiments with small squads in combat situations using the M14, AR-15, and Winchester's Light Weight Military Rifle (WLWMR).[47] The resulting study recommended adopting a lightweight rifle like the AR-15. In response, the Army declared that all rifles and machine guns should use the same ammunition and ordered full production of the M14.[21] However, advocates for the AR-15 gained the attention of Air Force Chief of Staff General Curtis LeMay. After testing the AR-15 with the ammunition manufactured by Remington that Armalite and Colt recommended, the Air Force declared that the AR-15 was its 'standard model' and ordered 8,500 rifles and 8.5 million rounds.[21] Advocates for the AR-15 in the Defense Advanced Research Projects Agency acquired 1,000 Air Force AR-15s and shipped them to be tested by the Army of the Republic of Vietnam (ARVN). The South Vietnam soldiers issued glowing reports of the weapon's reliability, recording zero broken parts while firing 80,000 rounds in one stage of testing, and requiring only two replacement parts for the 1,000 weapons over the entire course of testing. The report of the experiment recommended that the U.S. provide the AR-15 as the standard rifle of the ARVN, but Admiral Harry Felt, then Commander in Chief of Pacific Forces, rejected the recommendations on the advice of the U.S. Army.[21]
Throughout 1962 and 1963, the U.S. military extensively tested the AR-15. Positive evaluations emphasized its lightness, "lethality", and reliability.[21] However, the Army Materiel Command criticized its inaccuracy at longer ranges and lack of penetrating power at higher ranges.[48] In early 1963, the U.S. Special Forces asked and was given permission, to make the AR-15 its standard weapon. Other users included Army Airborne units in Vietnam and some units affiliated with the Central Intelligence Agency. As more units adopted the AR-15, Secretary of the Army Cyrus Vance ordered an investigation into why the weapon had been rejected by the Army. The resulting report found that Army Materiel Command had rigged the previous tests, selecting tests that would favor the M14 and choosing match grade M14s to compete against AR-15s out of the box.[21] At this point, the bureaucratic battle lines were well-defined, with the Army ordnance agencies opposed to the AR-15 and the Air Force and civilian leadership of the Defense Department in favor.[21]
In January 1963, Secretary of Defense Robert McNamara concluded that the AR-15 was the superior weapon system and ordered a halt to M14 production.[49] In late 1963, the Defense Department began mass procurement of rifles for the Air Force and special Army units. Secretary McNamara designated the Army as the procurer for the weapon with the Department, which allowed the Army ordnance establishment to modify the weapon as they wished. The first modification was the addition of a "manual bolt closure," allowing a soldier to ram in a round if it failed to seat properly. The Air Force, which was buying the rifle, and the Marine Corps, which had tested it both objected to this addition, with the Air Force noting, "During three years of testing and operation of the AR-15 rifle under all types of conditions the Air Force has no record of malfunctions that could have been corrected by a manual bolt closing device." They also noted that the closure added weight and complexity, reducing the reliability of the weapon. Colonel Harold Yount, who managed the Army procurement, would later state the bolt closure was added after direction from senior leadership, rather than as a result of any complaint or test result, and testified about the reasons: "the M-1, the M-14, and the carbine had always had something for the soldier to push on; that maybe this would be a comforting feeling to him or something."[50]
After modifications, the new redesigned rifle was subsequently adopted as the M16 Rifle:[51]
(The M16) was much lighter compared to the M14 it replaced, ultimately allowing soldiers to carry more ammunition. The air-cooled, gas-operated, magazine-fed assault rifle was made of steel, aluminum alloy, and composite plastics, truly cutting-edge for the time. Designed with full and semi-automatic capabilities, the weapon initially did not respond well to wet and dirty conditions, sometimes even jamming in combat. After a few minor modifications, the weapon gained in popularity among troops on the battlefield.[52][note 3]
Despite its early failures the M16 proved to be a revolutionary design and stands as the longest continuously serving rifle in US military history.[54] It has been adopted by many US allies and the 5.56×45 mm NATO cartridge has become not only the NATO standard but "the standard assault-rifle cartridge in much of the world."[55] It also led to the development of small-caliber high-velocity service rifles by every major army in the world.[56] It is a benchmark against which other assault rifles are judged.[56][note 4]
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In July 1960, General Curtis LeMay was impressed by a demonstration of the ArmaLite AR-15. In the summer of 1961, General LeMay was promoted to U.S. Air Force chief of staff and requested 80,000 AR-15s. However, General Maxwell D. Taylor, chairman of the Joint Chiefs of Staff, advised President John F. Kennedy that having two different calibers within the military system at the same time would be problematic and the request was rejected.[59] In October 1961, William Godel, a senior man at the Advanced Research Projects Agency, sent 10 AR-15s to South Vietnam. The reception was enthusiastic, and in 1962 another 1,000 AR-15s were sent.[60] United States Army Special Forces personnel filed battlefield reports lavishly praising the AR-15 and the stopping power of the 5.56 mm cartridge, and pressed for its adoption.[38]
The damage caused by the 5.56 mm bullet was originally believed to be caused by "tumbling" due to the slow 1 turn in 14-inch (360 mm) rifling twist rate.[61] However, any pointed lead core bullet will "tumble" after penetration into flesh, because the center of gravity is towards the rear of the bullet. The large wounds observed by soldiers in Vietnam were caused by bullet fragmentation created by a combination of the bullet's velocity and construction.[62] These wounds were so devastating that the photographs remained classified into the 1980s.[63]
However, despite overwhelming evidence that the AR-15 could bring more firepower to bear than the M14, the Army opposed the adoption of the new rifle.[49] U.S. Secretary of Defense Robert McNamara now had two conflicting views: the ARPA report[64] favoring the AR-15 and the Army's position favoring the M14.[38] Even President 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 reported that only the M14 was suitable for service, but Vance wondered about the impartiality of those conducting the tests. He ordered the Army Inspector General to investigate the testing methods used; the inspector general confirmed that the testers were biased toward the M14.
In January 1963, Secretary McNamara received reports that M14 production was insufficient to meet the needs of the armed forces and ordered a halt to M14 production.[38] 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 its adoption, despite receiving reports of several deficiencies, most notably the lack of a chrome-plated chamber.[65]
After modifications (most notably, the charging handle was re-located from under the carrying handle like the AR-10, to the rear of the receiver),[66] the newly redesigned rifle was renamed the Rifle, Caliber 5.56 mm, M16.[13][56] Inexplicably, the modification to the new M16 did not include a chrome-plated barrel. Meanwhile, the Army relented and recommended the adoption of the M16 for jungle warfare operations. However, the Army insisted on the inclusion of a forward assist to help push the bolt into battery if a cartridge failed to seat into the chamber. The Air Force, Colt, and Eugene Stoner believed that the addition of a forward assist was an unjustified expense. As a result, the design was split into two variants: the Air Force's M16 without the forward assist, and the XM16E1 with the forward assist for the other service branches.
In November 1963, McNamara approved the U.S. Army's order of 85,000 XM16E1s;[67] and to appease General LeMay, the Air Force was granted an order for another 19,000 M16s.[68] In March 1964, the M16 rifle went into production and the Army accepted delivery of the first batch of 2,129 rifles later that year, and an additional 57,240 rifles the following year.[13]
In 1964, the Army was informed that DuPont could not mass-produce the IMR 4475 stick powder to the specifications demanded by the M16. Therefore, Olin Mathieson Company provided a high-performance ball propellant. While the Olin WC 846 powder achieved the desired 3,300 ft (1,000 m) per second muzzle velocity, it produced much more fouling, which quickly jammed the M16's action (unless the rifle was cleaned well and often).[21]
In March 1965, the Army began to issue the XM16E1 to infantry units. However, the rifle was initially delivered without adequate cleaning kits[38] or instructions because advertising from Colt asserted that the M16's materials made the weapon require little maintenance, leading to a misconception that it was capable of self-cleaning.[69] Furthermore, cleaning was often conducted with improper equipment, such as insect repellent, water, and aircraft fuel, which induced further wear on the weapon.[70] As a result, reports of stoppages in combat began to surface.[38] The most severe problem was known as "failure to extract"—the spent cartridge case remained lodged in the chamber after the rifle was fired.[38][71] Documented accounts of dead U.S. troops found next to disassembled rifles eventually led to a Congressional investigation:[72]
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.[73]
In February 1967, the improved XM16E1 was standardized as the M16A1.[74] The new rifle had a chrome-plated chamber and bore to eliminate corrosion and stuck cartridges, and other minor modifications.[38] New cleaning kits, powder solvents, and lubricants were also issued. Intensive training programs in weapons cleaning were instituted including a comic book-style operations manual.[75] As a result, reliability problems were largely resolved and the M16A1 rifle achieved widespread acceptance by U.S. troops in Vietnam.[76]
In 1969, the M16A1 officially replaced the M14 rifle to become the U.S. military's standard service rifle.[77][78] In 1970, the new WC 844 powder was introduced to reduce fouling.[79]
Colt, H&R, and GM Hydramatic Division manufactured M16A1 rifles during the Vietnam War.[citation needed] M16s were produced by Colt until the late 1980s when FN Herstal (FN USA) began to manufacture them.[80]