In addition to the question of lethality of the 5.56mm cartridge, if you read Mark Bowden's book 'Black Hawk Down', there is a passage when one of the Delta Force shooters, (I forget which one) fire 4 rounds of a 5.56mm Tungsten round at one of the militia, who gets back up again and starts firing. His buddy who is carrying a M14 rifle fires one (that's ONE) 7.62x51 round at another Somali who falls down on the ground DEAD and this is in close quarters!.
7.62mm will always beat 5.56mm in combat situations, whether CQC or long-range, although a lot of armies don't seem to realise this. The B