I have heard those stories. I wouldnt argue with any GI who was there. I believe after a while they patched up the reliability problem.
Sure. After several thousand of us were needlessly killed or maimed and the war was over.
I had a older guy I was friends with who was in Vietnam and he said he was in a firefight and an NVA solider was charging him at fairly close range and he was hammering with his M16 and getting hits but he kept coming. Finally an old guy in the platoon had an M1 Carbine that had a selector on it, 3 rd. burst and full auto. He cut loose with the M1 on 3 rd. burst and he said it was like someone poleaxed the NVA goober right into the ground.
For most critters in North America the .223 should work, but if you want to absolutely make sure on them the .308 and the .308 for the two legged variety of critter.