The AR-15, sometimes known as the M-16, is an assault rifle that was first used in the Vietnam War. The U.S. military needed to compete with the Soviet Union's AK-47, so it selected this "accidental rifle," which was initially rife with problems and prone to jamming during combat, according to C.J. Chivers, war veteran, reporter, and author of "The Gun." But the rifle has had a slew of upgrades since then, and is the rifle most likely to be found in the hands of American soldiers on a modern battlefield.