United States Department of Defense - M16

M16 (more formally United States Rifle, Caliber 5.56 mm, M16) is the U.S. military designation for a family of rifles derived from the ArmaLite AR-15 and further developed by Colt starting in the mid-20th century.

The M16 rifle family including the M16/A1/A2/A3/A4 has been the primary infantry rifle of the United States military since the 1960s. With its variants, it has been in use by 15 NATO countries, and is the most produced firearm in its caliber. The M16 entered Army service in 1964.

The M16 is a lightweight, 5.56 mm caliber, air-cooled, gas-operated, magazine-fed rifle, with a rotating bolt, actuated by direct impingement gas operation. It is constructed of steel, aluminum and composite plastics.

The US Air Force M16 and the US Army's XM16E1 were the first versions fielded. The XM16E1 was soon standardized as the M16A1. The M16A1 was simply the M16 with a forward assist feature requested by the Army. All early versions fired the M193/M196 cartridge in semi- or full-automatic modes. This took place in the early 1960s, with major fielding by the Army in late 1964.Commercial AR-15s were first issued to Special Forces troops in the spring of 1964.
A U.S. soldier on exercises with an M16A1 and M40 Field Protective Mask: note the forward assist on the receiver and flash suppressor
A U.S. soldier on exercises with an M16A1 and M40 Field Protective Mask: note the forward assist on the receiver and flash suppressor

The M16A2 entered service in the 1980s and fired the NATO standard Belgian-designed M855/M856 cartridge. The M16A2 is a select fire rifle capable of semi-automatic or three-round bursts and incorporating design elements desired by the Marine Corps.These features included an adjustable rear-sight for windage and elevation, a 5/8 inch longer buttstock, heavier barrel, case deflector for left-handed shooters, and rounded handguards. Mode of fire is determined by using a selector switch on the side of the weapon. The M16A3 was essentially an M16A2 with an M16A1 fire control group; this had only limited procurement by the USN.

The M16A4 became standard issue for the United States Marine Corps during Operation Iraqi Freedom, increasingly replacing the earlier M16A2. In the United States Army, a combination of the M16A4 and M4 Carbine continues to replace existing M16A2 Rifles. The A4 incorporates a flattop receiver unit developed for the M4 Carbine, a handguard with four Picatinny rails for mounting optical sights, lasers, night vision devices, forward handgrips, removable carry handle and flashlights.

The M16 is primarily manufactured by Colt and Fabrique Nationale de Herstal, with variants produced by numerous countries around the world. Semi-automatic versions, generally referred to as "AR-15s" (because of Colt's purchase of the nomenclature from Armalite), are popular recreational firearms in the United States, with versions manufactured by a handful of larger manufacturers and over a dozen smaller concerns.

The M16 was an initial version first adopted in 1964 by the United States Air Force (USAF). The U.S. Army began to field the XM16E1 en masse in 1965 with most going to Vietnam. The US Marine Corps also adopted the system during this period. The XM16E1 was standardized as the M16A1 in 1967. This version remained the primary infantry rifle of the United States military from 1967 until the 1980s, when it was supplemented by the M16A2. During the early 1980s a roughly standardized load for this ammunition was adopted throughout NATO (see: 5.56 mm NATO).

The M16A3 is a fully-automatic variant of the M16A2, issued primarily within the United States Navy. The M16A2, in turn, is currently being supplemented by the M16A4, which incorporates the flattop receiver unit developed for the M4 Carbine, and Picatinny rail System. Previous versions of the weapon are still in stock and used primarily by reserve and National Guard units in the United States as well as by the U.S. Air Force.