I've read articles/news reports in which troops in Iraq criticized the 5.56 Nato round, saying it often takes 6,7, or 8 hits to put a Jihadist down for good (Obviously, I can't vouch for the veracity of these reports.).
I believe some Marines are now carrying the M-14 (7.62x51/.308 Winchester). IMO That speaks volumes.
Remember, our armed forced are prohibited the use of expanding/hollow point ammo. .223 ball doesn't make a very big hole.
That's part humanitarian and part practical. If you kill an enemy outright, you've taken one soldier off the field. If you wound him, you've taken out of the fight him and the two comrades it will take to carry him to safety.