m16

 Rifle, Caliber 5.56 mm, M16, is the United States military select-fire adaptation of the AR-15 rifle. The rifle was adapted for semi-automatic and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO cartridge. The rifle entered United States Army service and was deployed for jungle warfare operations in South Vietnam in 1963. becoming the U.S. military's standard service rifle of the Vietnam War by 1969

The M16 has also been widely adopted by other militaries around the world. Total worldwide 

production of M16s has been approximately 8 million, making it the most-produced firearm of its 5.56 mm caliber.  As of 2010, the U.S. Army is supplementing the M16 in combat units with the M4

carbine, which is a smaller version of the M16