Check out the posting at the Tactical Forums under Church of the Tactical Truth, USMC Lessons Learned (form the latest iraq war)&(sorry I'm not computer savy enough to post a link)here is a snippet:
5.56mm vs. 7.62 Lethality ~ 5.56mm “definitely answered the mail” and “as long as the shots were in the head or chest they went down” were typical quotes from several Marines; many who were previously very skeptical of 5.56mm ammunition. Most of the interviewed Marines who reported targets not going down and/or could still fight were referencing non-lethal shots to the extremities. There were reports of targets receiving shots in the vitals and not going down. These stories need not be described, but are of the rare superhuman occurrences that defy logic and caliber of round. Some Marines did ask about getting the heaver-grained 5.56mm rounds, up to 77 grain if possible.