The M16 (more formally Rifle, Caliber 5.56 mm, M16) is the United States military designation for the AR-15 rifle. 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 rifle fires the 5.56x45mm cartridge and can produce massive wounding and hydrostatic shock effects when the bullet impacts at high velocity and yaws in tissue leading to fragmentation and rapid transfer of energy. However, terminal effects can be unimpressive when the bullet fails to yaw or fragment in tissue.
The M16 entered United States Army service as the M16A1 and was put into action for jungle warfare in South Vietnam in 1963, becoming the standard U.S. rifle of the Vietnam War by 1969, replacing the M14 rifle in that role. The U.S. Army retained the M14 in CONUS, Europe, and South Korea until 1970. Since the Vietnam War, the M16 rifle family has been the primary infantry rifle of the U.S. military. With its variants, it has been in use by 15 NATO countries, and is the most produced firearm in its caliber.