Tonk: we found the VC to be tough customers when they were juiced up. When 'aimed fire' was not a luxury the 5.56 would not immediately put them down, but when we had the luxury of 'aimed fire', chest shots through the breast bone would stop them; those men we shot either stopped and fell over or began staggering blowing mouthfuls of blood, and dropped - they responded somewhat like chest shot Whitetail, if they got up at all after being hit or if they stayed on their feet.
When the VC and NVA would begin some of the 'human wave' assaults, non-fatal wounds with the 5.56 did not seem to have a immediate effect unless the rounds busted arm or leg bone. One VC literally lost his right arm to a 5.56 round and dropped his AK as a result, but stood there for a moment trying to figure out what had happened - when he bent down to pick up his rifle with his left hand and stood back up he was simply shot again. When they would come at us in waves we found our heavier 30 caliber machine guns, the 1919s and 1919A4s and M60s, would lay them down. In closer quarters the M3 and M3A 'Grease Guns' and Thompsons were the choice. With the M16, if you could place your shots into their chests they would go down - but sometimes their bandoleers or magazine pouches strapped across their chests would deflect the bullet into a non-fatal area, so you would just have to shoot again.
I have no doubt our troops in the middle east might be experiencing the same thing. jmtcw.