We were taught in the Army to fire a burst of 3.
AR15s & M16s had BURST settings (3 shots or praps 4 shots) a few years later than 1963.
The AR15 601 in 1963 had i think 3 settings.... SAFETY.... SEMI (ie a single shot per sqeez).... AUTO (ie fires when trigger sqeezed & keeps firing untill trigger released)(probably at least 2 shots minimum)(say 4 or 5 shots if accidentally sqeezed).
Plus it had i think 3 slamfire pseudo settings.... SAFETY(SLAMFIRE)(mightbe)(ie empties the magazine i think).... SEMI(SLAMFIRE)(ie single shot i think).... AUTO(SLAMFIRE)(ie empties the magazine i think).
The AR15 602 had a less massive firing pin & hence didnt have the 3 slamfire pseudo settings.