The M4 carbine is a 5.56×45mm NATO, gas-operated, magazine-fed carbine developed in the United States during the 1980s. It is a shortened version of the M16A2 assault rifle. The M4 is extensively used by the United States Armed Forces, with decisions to largely replace the M16 rifle in United States Army and United States Marine Corps combat units as the primary infantry weapon and service rifle. The M4 has been adopted by over 60 countries worldwide, and has been described as "one of the defining firearms of the 21st century".