Navy SEALs keep the M14 in inventory due to its ... The M14 was designed in 1954 and officially became the standard-issue rifle for the U.S. Armed Forces from 1959 to 1964, with over 1 million ...
If we’re going to go with length of service, then one could make the case for the M16/M4 series, which in one variant or another has been the Army’s standard issue rifle since 1965.