The M4 carbine is an assault rifle 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 U.S. military, which decided to largely replace the M16 rifle in U.S. Army and U.S. Marine Corps combat units as the primary infantry service rifle. The M4 has been adopted by over 60 countries, and has been described as "one of the defining firearms of the 21st century".