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The M16 is a type of assault rifle that fires NATO 5.56mm ammunition.

It was originally developed as the Armalite[?] AR-15, in the late 1950s and early 1960s by Eugene Stoner[?] of Armalite Systems, formerly of Costa Mesa, California. The AR-15 was initially adopted by the U.S. Special Forces[?], and later found favor with the general army. It was later produced by Colt[?], and other makers. The total quantity produced in all models world-wide has been about 7 million. It is one of a family of related weapons, including the AR-10, a relatively rare 7.62x51mm NATO rifle that recently returned to production, and a squad automatic weapon.

A major goal of the design was to make a lightweight rifle with lightweight ammunition, suited to modern warfare, to replace the heavy and less favored M14 rifle that was standard issue until 1962. See the doctrine section of assault rifle for more information.

The M16 is an ergonomic, angular, modernistic, and unusually lightweight rifle. It has a pistol grip, which aids intuitive pointing. The sights are specially designed to allow the user to simply dial in the range to the target without having to manually adjust the sights. The M16 type rifle is made of aluminum and plastics, except for the steel barrel and parts of the action. Most models weigh near 3Kg (6lbs), about a third less than comparable rifles.

One distinctive feature is a plastic or metal stock directly behind the action, which contains a recoil spring to reduce muzzle rise, especially during automatic fire. As a result, most users find the M16 type rifle easy to use without being intimidated by strong recoil, and the rife is very controllable. Because the recoil does not shift the point of aim, user fatigue is reduced.

Another distinctive feature is that the main sight is in the top of a carry handle on top of the receiver. The carry handle is a popular feature. Newer models have a "flattop" upper receiver to which the user can attach either a conventional carry handle/sighting system or numerous optical devices such as night vision[?] scopes.

The action is gas-operated, recocked by gases from a small hole in the barrel. The M16 design is so lightweight in part because it has a unique "direct drive" gas system. Hot gases from the barrel vent directly into the receiver to push the bolt carrier rearward, eliminating the need for a traditional operating rod and spring assembly. While this reduces the number of moving parts and results in a simpler design, maintenance can be a little tricky and the rifle is not quite as reliable as earlier U.S. rifles. Modern versions of the M16A2 with modern ammunition are very reliable.

A cocking lever was omitted from the earliest models to prevent entry of dirt, but it is included on the modern models along with a spring loaded dust cover.

In early models, the selective fire control selected either single-shot, or automatic. As designed the rate of automatic fire was about 700 rounds per minute, but a last-minute change to the gunpowder formula in the ammunition caused it to become very high, near 900 rounds per minute. Later, the Army decided this was a disadvantage. In the 1970s, the M16A1 was developed. In the first fire option, each pull of the trigger fires one shot. The other position allows three shots per trigger-pull. The U.S. Army performed years of experiments to discover and verify that three-shot groups were optimum, originally in order to develop a flechette rifle. Civilian models lack a selective fire control.

The magazine release is on the right side of the rifle but can be switched for left-handed users. Current military magazines have 30 rounds, and are sometimes taped in upside-down pairs to speed reloading. This practice is officially discouraged, because it increases the chance that the top of the magazine will be damaged or pick up dirt.

In early models, a low-twist rifling scheme gave muzzle velocities exceeding 3200fps, however, the bullet could tumble at long ranges. Modern rifles have a faster rifling twist, and the muzzle velocity is nearly as high, at 2900fps. The bullet is small caliber, 5.56mm (.223"), and often fragments when it strikes flesh. The combination of high velocity and a fragile small bullet is more likely to cause incapacitating injuries than death by hydrostatic shock. The relatively small bullet drifts more than heavier bullets at long ranges, but users can be trained to compensate.

It is advised to keep this weapon dry. A surprising minor weakness is that the barrel can wick water up into the barrel by capillary action. In this state, the weapon can misfire, possibly injuring the user.

Early U.S. users in the Vietnam war had numerous reliability problems. Some believe that this is because those users (who had allegedly been told that the gun required very little maintenance) had neglected maintenance and the neglected guns became extremely unreliable. However, other evidence points to subtle problems with compatibility between the ammunition and the early versions of the gun, such that even perfectly maintained and cleaned guns were unreliable.

The gunpowder of early version M16 ammunition was clean-burning, and the gun did not require chrome plating in the receiver area. It is widely believed that a last-minute change to the gunpowder formula was made shortly before the gun was introduced into service. While resulting in a higher muzzle velocity, it caused the weapon to foul much more quickly, and because it lacked plating, it would tend to jam.

The M16 has gone through several revisions since it was first adopted. After the M16A1 was adopted in the 1970s, user feedback and doctrinal changes led to the development of the M16A2, a further refinement of the design, in the 1980s.

The M16A2 assault rifle is the standard by which all military rifles of the future will be judged. This variant has fire modes of semi and three-round burst. The system incorporates an adjustable dual-aperture rear sight that corrects for both windage and elevation, a heavier barrel with 1-in-7 rifling, and an effective muzzle compensator to prevent muzzle climb during operation. The M16A2 is capable of firing all NATO standard 5.56mm ammunition and can fire 40mm grenades when equipped with the M203[?] Grenade Launcher. The M16A2 remains in service more than 20 years after its adoption by the U.S. Army in 1982. With user-friendly features and excellent accuracy out of the box, the M16A2 and subsequent revisions will remain the standard issue rifle of U.S. forces until either caseless ammunition[?] or laser weapons[?] become economically and practically feasible. Currently the U.S. Army has issued limited numbers of the newest variants, the M16A3 and M16A4, which incorporate a rail mounting system similar to the M4A1 Carbine.

In Vietnam, some soldiers were issued a unique version of the M16 called the XM-177[?] or CAR-15. The XM-177 had a shorter barrel (~26cm) and a telescoping stock, which made it substantially more compact and significantly handier. Numerous problems with muzzle flash and loud report resulted in Colt modifying the design to produce the XM-177E1 and XM-177E2 toward the end of the Vietnam conflict. The final XM-177E2 had a 29cm barrel with a long flash suppressor. This version became known as the "Commando" model and was issued in limited numbers to special forces, helicopter crews, Air Force pilots, officers, radio operators, artillerymen, and troops other than front line riflemen. The XM-177E2 is the forerunner of today's M4 Carbine, which was developed in the early 1990s, adopted officially in 1994, and was used with great success in the Balkans, the war on terror, and most recently in Iraq.

The M4/M4A1 5.56mm Carbine is a lightweight, gas operated, air cooled, magazine fed, selective rate, shoulder fired weapon with a collapsible stock. A shortened variant of the M16A2 rifle with a 36cm barrel, the M4 provides the individual soldier operating in close quarters the capability to engage targets at extended range with accurate, lethal fire. The M4 Carbine has "semi" and "auto" firing modes, but no three round burst. The M4 Carbine achieves over 80% commonality with the M16A2 Rifle and will replace all M3 .45 caliber submachine guns and selected M9 pistols and M16 rifle series. The M4A1 is favored by the U.S. Army's elite Delta Force[?], the Australian Special Forces, South Korea, and the infantry forces of several other countries. Capable of mounting the M203 Grenade Launcher, the M4A1's reduced size and weight offers a potent package for the urban warrior.

Despite being over 40 years old, the M16 design continues to be a mainstay of the American infantry forces. As a result of widespread adoption, popularity with the troops, and royalty-free manufacturing by Colt[?] and Fabrique Nationale[?], the M16 series will remain the rifle of the U.S. military forces for the foreseeable future. Newer designs by companies like Heckler & Koch, such as the G36[?] rifle, have still failed to catch the attention of the U.S. Department of Defense, although the competing models are making substantial inroads into the U.S. law enforcement market.

M16 is a Messier object in the constellation Serpens. It is also known as the Eagle Nebula[?].

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