Could be a number of factors. Remember there were a number of changes made to the M16 and it's ammo before they ever arrived in Vietnam to the soldiers. The powers that be at the time had such blind faith in the M16 that they sent them over to Vietnam without any cleaning kits!
Axxe55,
HAD the ammo supplied to the GIs in RVN been "in spec", the Army MIGHT have gotten away with NOT cleaning the M16 for a considerable period of time.
BUT, as the "supplied ammo" was NOT within specifications, the M16 became known as "the JAM-o-matic".
yours, satx