The M16 is a lightweight, 5.56 mm caliber, air-cooled, gas-operated, magazine-fed assault rifle, with a rotating bolt, actuated by direct impingement gas operation. The rifle is made of steel, aluminum, and composite plastics. The U.S. Air Force's rifle, the M16, and the U.S. Army's rifle, the XM16E1, were the first versions of the M16 rifle fielded. Soon, the Army standardized the XM16E1 as the M16A1 rifle, an M16 with a forward assist feature requested by the Army. All of the early versions were chambered to fire the M193/M196 cartridge in the semi-automatic and the automatic firing modes. This occurred in the early 1960s, with the Army issuing it in late 1964.

 

Commercial AR-15s were first issued to Special Forces troops in spring of 1964. The M16A2 rifle entered service in the 1980s, chambered to fire the standard NATO cartridge, the Belgian-designed M855/M856 cartridge. The M16A2 is a select-fire rifle (semi-automatic fire, three-round-burst fire) incorporating design elements requested by the Marine Corps: an adjustable, windage rear-sight; a stock 5/8-inch longer; heavier barrel; case deflector for left-hand shooters; and cylindrical hand guards. The fire mode selector is on the receiver's left side. The M16A2 is still the primary rifle in the U.S. Navy, Coast Guard, Air Force, and still is in heavy use in the Army and Marine Corps. The M16A3 rifle is an M16A2 rifle with an M16A1's fire-mode control (semi-automatic fire, automatic fire) used only by the U.S. Navy. The M16A4 rifle was standard issue for the United States Marine Corps in Operation Iraqi Freedom; it replaced the M16A2 in front line units.

 

In the U.S. Army, the M16A2 rifle is being supplemented with two rifle models, the M16A4 and the M4 Carbine, as the standard issue Assault rifle. The M16A4 rifle has a flat-top receiver developed for the M4 Carbine, a handguard with four Picatinny rails for mounting a sight, laser, night vision device, forward handgrip, removable handle, and a flashlight. The M16 rifle is principally manufactured by the Colt and the Fabrique Nationale de Herstal arms companies, with the variant rifles made elsewhere in the world. Versions for the U.S. military have also been made by H & R Firearms General Motors Hydramatic Division and most recently by Sabre Defence Industries.

 

The semi-automatic versions of the M16 rifle, generally called the "AR-15" (Colt bought the nomenclature from Armalite), are popular, recreational shooting rifles, with versions manufactured by other small and large manufacturers in the U.S.