The M16 was originally developed for air base security, not as infantry weapon. The classic assault rifle was developed in the 1950s by Eugene Stoner while he was tinkering at night in his garage. Colt bought the rights and produced the rifle for the U.S. government as the M16.
It entered widespread service in Vietnam in 1966, replacing the 7.62 mm M14 rifle. The M16A1 (shown here) was introduced in 1967. The main difference is the forward assist, installed to prevent the frequent jamming.