M16
M16A2, multi-sided view
Type	Assault rifle
Place of origin	United States
Service history
In service	1964–present[1]
Used by	See Users
Wars	See Conflicts
Production history
Designer	Eugene Stoner (AR-10)[2] L. James Sullivan (AR-15)[3]
Designed	1959[4]
Manufacturer	Colt's Manufacturing Company Daewoo Precision Industries FN Herstal Harrington & Richardson General Motors Hydramatic Division Elisco U.S. Ordnance
Produced	1963–present[1] [obsolete source]
No. built	c. 8 million as of 2011[1]
Variants	See List of Colt AR-15 and M16 rifle variants
Specifications (M16)
Mass	Unloaded: 6.37 lb (2.89 kg) (M16A1) 7.5 lb (3.40 kg) (M16A2 without magazine and sling)[5] 7.5 lb (3.40 kg) (M16A4) 8.81 lb (4.00 kg) (M16A4 loaded with 30 rounds and sling)[6]
Length	38.81 in (986 mm) (M16A1) 39.63 in (1,007 mm) (M16A2) 39.37 in (1,000 mm) (M16A4)
Barrel length	20 in (508 mm)
Cartridge	5.56×45mm NATO
Action	Gas-operated closed rotating bolt
Rate of fire	700–800 rounds/min cyclic sustained (M16A1)[7] 700-900 rounds/min cyclic sustained (M16A2, M16A3)[8] 800 rounds/min cyclic sustained (M16A4)[8]
Muzzle velocity	3,150 ft/s (960 m/s) (M855A1 round)[9]
Effective firing range	550 m (601 yd) (point target)[10] 800 m (875 yd) (area target)[11]
Maximum firing range	3,600 m (3,937 yd)
Feed system	STANAG magazine 20-round detachable box magazine 30-round detachable box magazine 40-round detachable box magazine 60-round detachable box magazine Beta C-Mag 100-round drum magazine
Sights	Iron sights: Rear: aperture; L-type flip Front: wing-protected post Various aiming optics

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 M16 entered US military service and in the following year was deployed for jungle warfare operations during the Vietnam War.^[12] In 1969, the M16A1 replaced the M14 rifle to become the US military's standard service rifle.^[13] The M16A1 incorporated numerous modifications including a bolt-assist, chrome-plated bore, protective reinforcement around the magazine release, and revised flash hider.^[12]

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.^[14] Adopted in July 1997, the M16A4 is the fourth generation of the M16 series. It is equipped with a removable carrying handle and Picatinny rail for mounting optics and other ancillary devices.^[15]

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.^[16][1] The US military has largely replaced the M16 in frontline combat units with a shorter and lighter version, the M4 carbine.^[17][18] 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 rifle is designated XM7.^[19]

History

Background

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.^[20] 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.^[21] However, early experiments with select-fire versions of the M1 Garand proved disappointing.^[22] During the Korean War, the select-fire M2 carbine largely replaced the submachine gun in US service^[23] and became the most widely used carbine variant.^[24] However, combat experience suggested that the .30 Carbine round was underpowered.^[25] American weapons designers concluded that an intermediate round was necessary, and recommended a small-caliber, high-velocity cartridge.^[26]

However, senior American commanders, having faced fanatical enemies and experienced major logistical problems during World War II and the Korean War,^[27][28] 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.^[29] This culminated in the development of the 7.62×51 mm NATO cartridge.^[30]

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.^[31] The AR-10 featured an innovative straight-line barrel/stock design, forged aluminum alloy receivers, and with phenolic composite stocks.^[32] It had rugged elevated sights, an oversized aluminum^{[note 1]} flash suppressor and recoil compensator, and an adjustable gas system.^[34] 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.^[31] For a 7.62 mm NATO rifle, the AR-10 was incredibly lightweight at only 6.85 lb (3.11 kg) empty.^[35] 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.^[36] In the end, the U.S. Army chose the T44, now named the M14 rifle,^[30] which was an improved M1 Garand with a 20-round magazine and automatic fire capability.^{[37][note 2]} The U.S. also adopted the M60 general-purpose machine gun (GPMG).^[30] 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.^[39] And, while the M2 carbine offered a high rate of fire, it was under-powered and ultimately outclassed by the AK-47.^[40] 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.^[41]

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.^[42] 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.^[43]

This request ultimately resulted in the development of a scaled-down version of the Armalite AR-10, named the ArmaLite AR-15.^[44] The AR-15 was first revealed by Eugene Stoner at Fort Benning in May 1957.^[45] 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.^[20]

In 1958, the Army's Combat Developments Experimentation Command ran experiments with small squads in combat situations using the M14, AR-15, and another rifle designed by Winchester. 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.^[20] 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.^[20] 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.^[20]

Throughout 1962 and 1963, the U.S. military extensively tested the AR-15. Positive evaluations emphasized its lightness, "lethality", and reliability.^[20] However, the Army Materiel Command criticized its inaccuracy at longer ranges and lack of penetrating power at higher ranges.^[46] 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.^[20] 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.^[20]

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.^[47] 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."^[48]

After modifications, the new redesigned rifle was subsequently adopted as the M16 Rifle:^[49]

(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.^{[50][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.^[52] 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."^[53] It also led to the development of small-caliber high-velocity service rifles by every major army in the world.^[54] It is a benchmark against which other assault rifles are judged.^{[54][note 4]}

Adoption

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.^[57] 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.^[58] 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.^[37]

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.^[59] 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.^[60] These wounds were so devastating that the photographs remained classified into the 1980s.^[61]

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.^[47] U.S. Secretary of Defense Robert McNamara now had two conflicting views: the ARPA report^[62] favoring the AR-15 and the Army's position favoring the M14.^[37] 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.^[37] 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.^[63]

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),^[64] the newly redesigned rifle was renamed the Rifle, Caliber 5.56 mm, M16.^[12][54] 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;^[65] and to appease General LeMay, the Air Force was granted an order for another 19,000 M16s.^[66] 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.^[12]

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).^[20]

In March 1965, the Army began to issue the XM16E1 to infantry units. However, the rifle was initially delivered without adequate cleaning kits^[37] 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.^[67] Furthermore, cleaning was often conducted with improper equipment, such as insect repellent, water, and aircraft fuel, which induced further wear on the weapon.^[68] As a result, reports of stoppages in combat began to surface.^[37] The most severe problem was known as "failure to extract"—the spent cartridge case remained lodged in the chamber after the rifle was fired.^[37][69] Documented accounts of dead U.S. troops found next to disassembled rifles eventually led to a Congressional investigation:^[70]

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.^[71]

In February 1967, the improved XM16E1 was standardized as the M16A1.^[72] The new rifle had a chrome-plated chamber and bore to eliminate corrosion and stuck cartridges, and other minor modifications.^[37] 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.^[73] As a result, reliability problems were largely resolved and the M16A1 rifle achieved widespread acceptance by U.S. troops in Vietnam.^[74]

In 1969, the M16A1 officially replaced the M14 rifle to become the U.S. military's standard service rifle.^[75][76] In 1970, the new WC 844 powder was introduced to reduce fouling.^[77]

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.^[78]

Reliability

During the early part of its service, the M16 had a reputation for poor reliability and a malfunction rate of two per 1000 rounds fired.^[79] The M16's action works by passing high-pressure propellant gasses, tapped from the barrel, down a tube and into the carrier group within the upper receiver. The gas goes from the gas tube, through the bolt carrier key, and into the inside of the carrier where it expands in a donut-shaped gas-piston cylinder. Because the bolt is prevented from moving forward by the barrel, the carrier is driven to the rear by the expanding gases and thus converts the energy of the gas to the movement of the rifle's parts. The back part of the bolt forms a piston head and the cavity in the bolt carrier is the piston sleeve. While the M16 is commonly said to use a direct impingement system, this is wrong, and it is instead correct to say it uses an internal piston system.^[80] This system is however ammunition specific, since it does not have an adjustable gas port or valve to adjust the weapon to various propellant and projectile or barrel length specific pressure behavior.

The M16 operating system designed by Stoner is lighter and more compact than a gas-piston design. However, this design requires that combustion byproducts from the discharged cartridge be blown into the receiver as well. This accumulating carbon and vaporized metal build-up within the receiver and bolt carrier negatively affects reliability and necessitates more intensive maintenance on the part of the individual soldier. The channeling of gasses into the bolt carrier during operation increases the amount of heat that is deposited in the receiver while firing the M16 and causes the essential lubricant to be "burned off". This requires frequent and generous applications of appropriate lubricant.^[81] Lack of proper lubrication is the most common source of weapon stoppages or jams.^[82]

The original M16 fared poorly in the jungles of Vietnam and was infamous for reliability problems in harsh environments. Max Hastings was very critical of the M16's general field issue in Vietnam just as grievous design flaws were becoming apparent. He further states that the Shooting Times experienced repeated malfunctions with a test M16 and assumed these would be corrected before military use, but they were not. Many marines and soldiers were so angry with the reliability problems they began writing home and on 26 March 1967, the Washington Daily News broke the story.^[83] Eventually, the M16 became the target of a congressional investigation.^{[note 5]}

The investigation found that:^[12]

• The M16 was issued to troops without cleaning kits or instructions on how to clean the rifle.
• The M16 and 5.56×45mm cartridge was tested and approved with the use of a DuPont IMR8208M extruded powder, which was switched to Olin Mathieson WC846 ball powder which produced much more fouling, which quickly jammed the action of the M16 (unless the gun was cleaned well and often).
• The M16 lacked a forward assist (rendering the rifle inoperable when it failed to go fully forward).
• The M16 lacked a chrome-plated chamber, which allowed corrosion problems and contributed to case-extraction failures (which was considered the most severe problem and required extreme measures to clear, such as inserting the cleaning rod down the barrel and knocking the spent cartridge out).

When these issues were addressed and corrected by the M16A1, the reliability problems decreased greatly.^[72] According to a 1968 Department of Army report, the M16A1 rifle achieved widespread acceptance by U.S. troops in Vietnam.^[85] "Most men armed with the M16 in Vietnam rated this rifle's performance high, however, many men entertained some misgivings about the M16's reliability. When asked what weapon they preferred to carry in combat, 85 percent indicated that they wanted either the M16 or its carbine-length version, the XM177E2." Also, "the M14 was preferred by 15 percent, while less than one percent wished to carry either the Stoner rifle, the AK-47, the carbine or a pistol."^[51] In March 1970, the "President's Blue Ribbon Defense Panel" concluded that the issuance of the M16 saved the lives of 20,000 U.S. servicemen during the Vietnam War, who would have otherwise died had the M14 remained in service.^[86] However, the M16 rifle's reputation has suffered as of 2011.^[87]

Another underlying cause of the M16's jamming problem was identified by ordnance staff that discovered that Stoner and ammunition manufacturers had initially tested the AR-15 using DuPont IMR8208M extruded (stick) powder. Later ammunition manufacturers adopted the more readily available Olin Mathieson WC846 ball powder. The ball powder produced a longer peak chamber pressure with undesired timing effects. Upon firing, the cartridge case expands and seals the chamber (obturation). When the peak pressure starts to drop the cartridge case contracts and then can be extracted. With ball powder, the cartridge case was not contracted enough during extraction due to the longer peak pressure period. The ejector would then fail to extract the cartridge case, tearing through the case rim, and leaving an obturated case behind.^[88]

After the introduction of the M4 carbine, it was found that the shorter barrel length of 14.5 inches also harms the reliability, as the gas port is located closer to the chamber than the gas port of the standard length M16 rifle: 7.5 inches instead of 13 inches.^[89] This affects the M4's timing and increases the amount of stress and heat on the critical components, thereby reducing reliability.^[89] In a 2002 assessment, the USMC found that the M4 malfunctioned three times more often than the M16A4 (the M4 failed 186 times for 69,000 rounds fired, while the M16A4 failed 61 times).^[90] Thereafter, the Army and Colt worked to make modifications to the M4s and M16A4s to address the problems found.^[90] In tests conducted in 2005 and 2006 the Army found that on average, the new M4s and M16s fired approximately 5,000 rounds between stoppages.^[90][91]

In December 2006, the Center for Naval Analyses (CNA) released a report on U.S. small arms in combat. The CNA conducted surveys on 2,608 troops returning from combat in Iraq and Afghanistan over the past 12 months. Only troops who had fired their weapons at enemy targets were allowed to participate. 1,188 troops were armed with M16A2 or A4 rifles, making up 46 percent of the survey. 75 percent of M16 users (891 troops) reported they were satisfied with the weapon. 60 percent (713 troops) were satisfied with handling qualities such as handguards, size, and weight. Of the 40 percent dissatisfied, most were with its size. Only 19 percent of M16 users (226 troops) reported a stoppage, while 80 percent of those that experienced a stoppage said it had little impact on their ability to clear the stoppage and re-engage their target. Half of the M16 users experienced failures in their magazines to feed. 83 percent (986 troops) did not need their rifles repaired while in the theater. 71 percent (843 troops) were confident in the M16's reliability, defined as a level of soldier confidence their weapon will fire without malfunction, and 72 percent (855 troops) were confident in its durability, defined as a level of soldier confidence their weapon will not break or need repair. Both factors were attributed to high levels of soldiers performing their maintenance. 60 percent of M16 users offered recommendations for improvements. Requests included greater bullet lethality, newly built instead of rebuilt rifles, better-quality magazines, decreased weight, and a collapsible stock. Some users recommended shorter and lighter weapons such as the M4 carbine.^[92] Some issues have been addressed with the issuing of the Improved STANAG magazine in March 2009,^[93][94] and the M855A1 Enhanced Performance Round in June 2010.^[95]

In early 2010, two journalists from The New York Times spent three months with soldiers and Marines in Afghanistan. While there, they questioned around 100 infantry troops about the reliability of their M16 rifles, as well as the M4 carbine. The troops did not report reliability problems with their rifles. While only 100 troops were asked, they engaged in daily fighting in Marja, including at least a dozen intense engagements in Helmand Province, where the ground is covered in fine powdered sand (called "moon dust" by troops) that can stick to firearms.^[96] Weapons were often dusty, wet, and covered in mud. Intense firefights lasted hours with several magazines being expended. Only one soldier reported a jam when his M16 was covered in mud after climbing out of a canal. The weapon was cleared and resumed firing with the next chambered round. Furthermore, the Marine Chief Warrant Officer responsible for weapons training and performance of the Third Battalion, Sixth Marines, reported that "We've had nil in the way of problems; we've had no issues", with his battalion's 350 M16s and 700 M4s.^[96]

Design

The M16 is a lightweight, 5.56 mm, air-cooled, gas-operated, magazine-fed assault rifle, with a rotating bolt. 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.

The M16 internal piston action was derived from the original ArmaLite AR-10 and ArmaLite AR-15 actions. This internal piston action system designed by Eugene Stoner is commonly called a direct impingement system, but it does not use a conventional direct impingement system. In U.S. patent 2,951,424, the designer states: ″This invention is a true expanding gas system instead of the conventional impinging gas system.″^[97] The gas system, bolt carrier, and bolt-locking design is ammunition specific, since it does not have an adjustable gas port or valve to adjust the weapon to various propellant and projectile or barrel length specific pressure behavior.

The M16A1 was especially lightweight at 7.9 pounds (3.6 kg) with a loaded 30-round magazine.^[98] This was significantly less than the M14 that it replaced at 10.7 pounds (4.9 kg) with a loaded 20-round magazine.^[99] It is also lighter when compared to the AKM's 8.3 pounds (3.8 kg) with a loaded 30-round magazine.^[100]

The M16A2 weighs 8.8 lb (4.0 kg) loaded with a 30-round magazine, 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.

Barrel

Early model M16 barrels had a rifling twist of four grooves, right-hand twist, one turn in 14 inches (1:355.6 mm or 64 calibers) bore—as it was the same rifling as used by the .222 Remington sporting cartridge. After finding out that under unfavorable conditions, military bullets could yaw in flight at long ranges, the rifling was soon altered. Later M16 models and the M16A1 had an improved rifling with six grooves, right-hand twist, one turn in 12 inches (1:304.8 mm or 54.8 calibers) for increased accuracy and was optimized to adequately stabilize the M193 ball and M196 tracer bullets. M16A2 and current models are optimized for firing the heavier NATO SS109 ball and long L110 tracer bullets and have six grooves, right-hand twist, one turn in 7 in (1:177.8 mm or 32 calibers).^[16][101]

M193 ball and M196 tracer bullets may be fired in a rifle with a one turn in 7 in (1:177.8 mm or 32 calibers) twist barrel. NATO SS109 ball and L110 tracer bullets should only be used in emergency situations at ranges under 90 m (98 yd) with a one turn in 12 inches (1:304.8 mm or 54.8 calibers) twist, as this twist is insufficient to stabilize these projectiles.^[102] Weapons designed to adequately stabilize both the M193 or SS109 projectiles (like civilian market clones) usually have a six-groove, right-hand twist, one turn in 9 inches (1:228.6 mm or 41.1 calibers) or one turn in 8 inches (1:203.2 mm or 36.5 calibers) bore, although other and 1:7 inches twist rates are available as well.

Recoil

The (M16's) Stoner system provides a very symmetric design that allows straight-line movement of the operating components. This allows recoil forces to drive straight to the rear. Instead of connecting or other mechanical parts driving the system, high-pressure gas performs this function, reducing the weight of moving parts and the rifle as a whole.^[80]

The M16 uses a "straight-line" recoil design, where the recoil spring is located in the stock directly behind the action,^[75] and serves the dual function of operating spring and recoil buffer.^[75] The stock being in line with the bore also 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. In addition, current model M16 flash-suppressors also act as compensators to reduce recoil further.^[98]

Notes: Free recoil is calculated by using the rifle weight, bullet weight, muzzle velocity, and charge weight.^[103] It is that which would be measured if the rifle were fired suspended from strings, free to recoil.^[103] A rifle's perceived recoil is also dependent on many other factors which are not readily quantified.^[103]

Sights

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 AR-10 design, where the carrying handle contained a rear sight that could be set for specific range settings and also served to protect the charging handle.^[75] The M16 carry handle also provided mounting groove interfaces and a hole at the bottom of the handle groove for mounting a Colt 3×20 telescopic sight featuring a Bullet Drop Compensation elevation adjustment knob for ranges from 100 to 500 yd (91 to 457 m). This concurs with the pre-M16A2 maximum effective range of 460 m (503 yd).^[104] The Colt 3×20 telescopic sight was factory adjusted to be parallax-free at 200 yd (183 m).^[105][106] In Delft, the Netherlands Artillerie-Inrichtingen produced a roughly similar 3×25 telescopic sight for the carrying handle mounting interfaces.^[107]

The M16 elevated iron sight line has a 19.75 in (502 mm) sight radius.^[16] As the M16 series rear sight, front sight and sighting in targets designs were modified over time and non-iron sight (optical) aiming devices and new service ammunition were introduced zeroing procedures changed.^[108]

The standard pre-M16A2 "Daylight Sight System" uses an AR-15-style L-type flip, two aperture rear sight featuring two combat settings: short-range 0 to 300 m (0 to 328 yd) and long-range 300 to 400 m (328 to 437 yd), marked 'L'.^[109] The pre-M16A2 "Daylight Sight System" short-range and long-range zeros are 250 and 375 m (273 and 410 yd) with M193 ammunition.^[109] The rear sight features a windage drum that can be adjusted during zeroing with about 1 MOA increments. The front sight is a tapered round post of approximately 0.0625 in (1.59 mm) diameter adjustable during zeroing in about 1 MOA increments. A cartridge or tool is required to (re)zero the sight line.^[110][108]

An alternative pre-M16A2 "Low Light Level Sight System", includes a front sight post with a weak light source provided by tritium radioluminescence in an embedded small glass vial and a two aperture rear sight consisting of a 2 mm (0.079 in) diameter aperture marked 'L' intended for normal use out to 460 m (503 yd) and a 7 mm (0.276 in) diameter large aperture for night firing.^[109] Regulation stipulates the radioluminescent front sight post must be replaced if more than 144 months (12 years) elapsed after manufacture.^[111] The "Low Light Level Sight System" elevation and windage adjustment increments are somewhat coarser compared to the "Daylight Sight System".^[109]

With the advent of the M16A2, a less simple fully adjustable rear sight was added, allowing the rear sight to be dialed in with an elevation wheel for specific range settings between 300 and 800 m (328 and 875 yd) in 100 m increments and to allow windage adjustments with a windage knob without the need of a cartridge or tool. The unmarked approximately 0.070 in (1.78 mm) diameter aperture rear sight is for normal firing situations, zeroing and with the elevation knob for target distances up to 800 meters. The downsides of relatively small rear sight apertures are less light transmission through the aperture and a reduced field of view. A new larger approximately 0.2 in (5.08 mm) diameter aperture, marked '0-2' and featuring a windage setting index mark, offers a larger field of view during battle conditions and is used as a ghost ring for quick target engagement and during limited visibility. When flipped down, the engraved windage mark on top of the '0-2' aperture ring shows the dialed in windage setting on a windage scale at the rear of the rear sight assembly. When the normal use rear aperture sight is zeroed at 300 m with SS109/M855 ammunition, first used in the M16A2, the '0-2' rear sight will be zeroed for 200 m. The front sight post was widened to approximately 0.075 in (1.91 mm) diameter and became square and became adjustable during zeroing in about 1.2 MOA increments.^[112][113]

The M16A4 omitted the carrying handle and rear sight assembly on top of the receiver. Instead, it features a MIL-STD-1913 Picatinny railed flat-top upper receiver for mounting various optical sighting devices or a new detachable carrying handle and M16A2-style rear sight assembly. The current U.S. Army and Air Force issue M4A1 Carbine comes with the M68 Close Combat Optic and Back-up Iron Sight.^[114] The U.S. Marine Corps uses the ACOG Rifle Combat Optic^[115][116] and the U.S. Navy uses the EOTech Holographic Weapon Sight.^[117]

Range and accuracy

The M16 rifle is considered to be very accurate for a service rifle.^[118] Its light recoil, high-velocity and flat trajectory allow shooters to take headshots out to 300 meters.^[119] Newer M16s use the newer M855 cartridge increasing their effective range to 600 meters.^[16] They are more accurate than their predecessors and are capable of shooting 1–3-inch groups at 100 yards.^{[120][note 6]} "In Fallujah, Iraq Marines with ACOG-equipped M16A4s created a stir by taking so many headshots that until the wounds were closely examined, some observers thought the insurgents had been executed."^[122] The newest M855A1 EPR cartridge is even more accurate and during testing "...has shown that, on average, 95 percent of the rounds will hit within an 8 × 8-inch (20.3 × 20.3 cm) target at 600 meters."^[123]