State side and in Europe yes, many where fixed to semi. But there is tons of footage of guys spraying away with their M14's. The locking to semi was never as far as I know a force wide thing. Certain units did do this but I would be totally shocked if the bulk of troops deployed to a close range jungle fight with M14's locked to semi auto only. The M14 like all of this era's battle rifles sucked on full auto. But this does give you 2 things semi does not. Clearing bunkers and rooms and suppression in a counter ambush type of situation close in. This is something that if a unit did deploy like this the unit armorer can swap back easy and a GI will do whatever they think will give them an edge. Crap, even M16's and M4's as of a few years back that had 3 round burst only it was well known how to disable the burst with a paper clip to get full auto instead. Troops back then would find a way. One way or another even if the higher ups in the unit where idiots.
Make the M14 for non specialized uses select fire. Make it suck on full auto like it should. An M14e2 would also be nice for a machine gun. Its sucked and was not used much as a squad MG, but it was out there. If M3 greese guns are everywhere in game might as well have an M14e2 as well.