Animation of M-14 rifle operation.
The M-14 was the US military service rifle from 1959 to 1970. As issued it was semi-automatic operation only. Although some were issued with full-auto conversion installed that allowed full-auto fire by flipping a selector level. These rifles were designated as the M-14A1.

The M-14 saw extensive use in Vietnam, where it served well. Despite the rifles heavy weight and excessive length - which made it clumsy for combat use in the jungles of Vietnam - the powerful round was a very good "jungle buster" as it would penetrate foliage and small trees that smaller rounds could not. It was superseded by the 5.56 caliber M-16 in the late 1960's.
http://en.wikipedia.org/wiki/M14_rifle#M14E1