M16 (rifle)

M16 is the U.S. Military designation for a family of rifles derived from the Armalite AR-15. It is an assault rifle which fires NATO standard 5.56 mm ammunition. It has been the primary infantry rifle of the United States military since 1967, is in use by 15 NATO countries, and has been the most produced firearm in its caliber.

Variants

M16

Variant originally adopted by the USAF. Effectively identical to the AR-15.

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XM16E1 and M16A1

The prototype army-version, XM16E1, was different in appearance from the pre-production AR-15, adding the forward assist. The M16A1 was the finalized production model. To address issues raised by the XM16E1's testing cyle, a "bird-cage" flash hider replaced the XM16E1's three-pronged flash hider, which was too easy for foreign material to get into and which caught on twigs and leaves. Soon after its adoption, barrels with chromed chambers and later fully chromed bores were introduced for the M16A1 to reduce the number of malfunctions due to fouling and corrosion.

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M16A2

Modifications to the M16A2 were more 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. 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 SS-109 rounds. The flash hider 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. The front grip was modified from the original triangular shape to a round one, which better fit smaller hands. The new handguards were also symmetrical so that armories didn't need separate left and right spares. The buttstock was redesigned to be longer and stronger. In fact, the new buttstock is said to be ten times stronger than the original due to advances in plastics and design. The heavier bullet has a reduced muzzle velocity from 3,200 feet per second (975 m/s) in the earlier models, to about 2,900 feet per second (875 m/s) in the A2. A special spent case deflector was incorporated into the upper receiver to the rear of the ejection port to prevent spent cases from striking left-handed users.

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The action was also modified, replacing the fully automatic setting with a three-round burst setting. When using a fully automatic weapon, poorly trained troops often hold down the trigger and "spray" when under fire. The US Army concluded that three-shot groups provide an optimum combination of ammunition conservation, accuracy and firepower. Recently, thinking has shifted away from the three-round burst setting and back to fully automatic fire. Soldiers today are better trainied, more disciplined, and enjoy more secure supply structures than they did in previous conflicts. There are also mechanical flaws in the M16A2 burst mechanism. The trigger group does not reset when the trigger is released. If a soldier lets go of the trigger between the second and third round of the burst, for example, the next trigger pull would only result in a single shot. Even in semi-automatic mode, the trigger group mechanism affects weapon handling. With each round fired, the trigger group cycles through one of the three staqes of the burst mechanism. The trigger pull at each of these stages is slightly different, detracting from accuracy.

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All together, the M16A2s new features added weight and complexity to the M16 while simultaneously decreasing barrel life and removing the fully automatic setting. In addition to the poor trigger mechanism, critics point out that neither of the rear sight apertures is ideally sized. The small aperture is too small, making quick acquisition of the front site post difficult, and the large aperture is too large, resulting in decreased accuracy. To make matters worse, the rear sight apertures are not machined to be on the same plane. In other words, the point of impact changes when the user changes from one aperture to the other. The rear sight's range adjustment feature is rarely used in combat. In fact, only the Marines train to use this features. Soldiers tend to leave the rear sight on its lowest range setting: 300 meters.

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XM177, M733 and M4 carbine versions

In Vietnam, some soldiers were issued a carbine version of the M16 called the XM177. The XM177 had a shorter barrel (~260 mm) 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 XM177 and the U.S. Army's XM177E1 variants differed over the latter?s inclusion of a forward assist. The final XM177E2 had a 290 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, but incorrectly, referenced as the CAR-15. The variants were issued in limited numbers to special forces, helicopter crews, Air Force pilots, officers, radio operators, artillerymen, and troops other than front line riflemen.

Related Topics:
Carbine - XM177 - XM148

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The XM177E2 led to the M733, essentially the same gun with an M16A2 receiver and internal improvements and a flash hider/compensator to reduce perceived recoil. The M4 Carbine was developed from these designs in the early 1990s, with a 14.5 inch (368 mm) barrel and a "flat top" railed receiver. 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, the 2000s War on Terrorism, and in Iraq.

Related Topics:
Balkans - War on Terrorism - Iraq

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A Firing Port Weapon or FPW was also developed to work with the Bradley IFV, designated the M231.

Related Topics:
Bradley IFV - M231

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M16A3

The M16A3 was 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 the SEALs. It features a Safe-Semi-Auto (S-1-F) trigger group like that of the M16A1.

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Some confusion continues to exist regarding the M16A3. It is often described as the fully-automatic version of the M16A4. Descriptions of the M16A3 that claim that it shares the M16A4's Picatinny rail are incorrect. This misunderstanding most likely stems from the usage of the A2 and A3 designations by civilian manufacturers to differentiate between A2-style fixed carry handles and Picatinny rail versions.

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M16A4

The M16A4, now standard issue for frontline U.S. Army and USMC units, replaces the combination fixed carry handle/rear iron sight with a M1913 Picatinny rail, allowing for the rifle to be equipped with a carry handle and/or most military and consumer scopes or sighting systems. All of the U.S. Marine Corps' M16A4s are equipped with a Knight's Armament Company M5 RAS handguard, allowing vertical grips, lasers, tatical lights, and other accessories to be attached. US Army M16A4s also often feature the KAC M5 RAS. In US Army Field Manuals, M16A4s fitted with the RAS are somtimes referred to as M16A4 MWS or Modular Weapon System.

Related Topics:
M1913 Picatinny rail - Knight's Armament Company

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Mk 4 Mod 0

A variant of the M16A1 produced for the US Navy SEALs during the conflict in Vietnam, the Mk 4 Mod 0 differed from the basic A1 primarily in being designed specifically to work with the US Army's Human Engineering Laboratory's sound suppressor, and optimized for maritime operations. Most of the operating parts of the rifle were coated in Kal-Guard, a quarter-inch hole 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 without damage. The HEL sound suppressor vented gas directly from the action, and required modification to the gas system, but also to the receiver to deflect the blast of gas from the shooter's face. The US Army designated the suppressor system as the M4A, while the US Navy designated the suppressor as the Mk 2 Mod 0. The suppressor's complexity meant that once the simple Sionics reflex suppressors, the MAW-A1s were available, it quickly fell by the wayside.

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