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M16 also refers to the Eagle Nebula in astronomy.

The M16 is an assault rifle which fires NATO 5.56mm ammunition. This has been the standard infantry weapon, of the US Army, for ~40 years. It is manufactured by Colt and Fabrique Nationale; with variants produced by Heckler und Koch.

Table of contents
1 Overview
2 United States Army Usage
3 Design
4 Maintenance
5 Vietnam


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.

United States Army Usage

The gun is often argued to have various technical problems; and there is a general preference for the M-4 Carbine (an M16 variant); however, since the M-4 is also seen as somewhat archiac; it is believed that the M16 will eventually be replaced by a proposed "Objective Individual Combat Weapon" or XM8.


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.

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. Early models were especially lightweight at 3.2 kg (~7.5 lbs), about 30% less than older 7.62mm "battle rifles" of the 1950s and 1960s. New models weigh more (~8.5 lbs) because of the development of the "heavy" barrel used to increase accuracy. The gun is 40 inches in length.

One distinctive feature is a plastic or metal stock directly behind the action, which contains a recoil spring that serves the dual function of operating spring and recoil buffer. The spring acts 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 rifle is very controllable. Because recoil does not significantly shift the point of aim, user fatigue is reduced. This reduction in recoil coincided well with the entry of women into the Army.

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, cocked 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 forward assist lever (to manually close the action in the event of a failure to feed) 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 M16A2 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 selective fire capability. If the trigger is released before all three rounds are fired, the next burst will only fire the remaining shots from the previous burst.

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. Aftermarket double magazine clips are available to assist in this area.

In early models, a low-twist rifling scheme gave muzzle velocities exceeding 3200 fps, however, the bullet could tumble at long ranges. Modern rifles have a faster rifling twist, and the muzzle velocity is nearly as high, at 2900 fps or more. 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.

The M16 has gone through several revisions since it was first adopted. The first revision was minor—the replacement of the original "pronged" flash suppressor with a "birdcage" model because of complaints that the pronged version snagged on brush in Vietnam. 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 to increase accuracy, 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.


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.

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 (~26 cm) 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 29 cm 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.

See also: M4 Carbine