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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 rare .30 calibre 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. See the doctrine section of assault rifle for more information.

It is an angular, modernistic, unusually light-weight rifle. It has a pistol-grip, which aids intuitive pointing. It is made of aluminum and plastics, except for the barrel and parts of the action. Most models weigh near 3Kg (6lbs), about a third less than comparable rifles.

A distinctive feature is a plastic or metal stock directly behind the action. This placement prevents the recoil during automatic fire from raising the barrel uncontrollably. Recoil is very light, and since the recoil does not shift the point of aim, user fatigue is reduced. The stock also contains a large return spring for the bolt carrier.

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.

The action is gas-operated, recocked by gases from a small hole in the barrel.

A cocking lever was omitted from the earliest models to prevent entry of dirt, but it is on the right side of modern models.

In early models, the selective fire control selected either single-shot, or automatic. The rate of automatic fire was very high, near 900 rounds per minute. Later, the Army decided this was a disadvantage. In the 1970s, the M-16A was developed. In the one-dot position, each pull of the trigger fires one shot. The other position shoots 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 usually lack a selective fire control.

The magazine release is on the front of the trigger guard. Most military magazines have 30 rounds, and are frequently taped in upside-down pairs to speed reloading. This practice is often 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 3000fps, however, the bullet could tumble at long ranges. Modern rifles have a stronger rifling twist, and the muzzle velocity is merely high, near 2400fps. The bullet is small calibre, 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.

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 will 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 been told that the gun required very little or no maintenance, so that they 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 plating in the receiver area. However, 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.

Modern versions of the M16 with modern ammunition are reliable.