The M16 rifle series is a family of assault rifles that fire NATO-standard 5.56 mm ammunition. Since 1967, the M16 has been the primary infantry weapon of the U.S. Army and has also been adopted by at least 15 NATO countries. It is the most widely produced model among rifles of the same caliber. The designation ‘M16’ was assigned by the U.S. military to the weapon developed from the Armalite AR-15.