theres a bunch of dffernt things figured in. all modern millitary rounds are small and fast moving. nato, china, russia... its light weight(both for transportation and soldier combat load), deadly and works well with the Geneva convention requirement of full metal jacket rounds. hunting rounds are lead tiped to kill things, FMJ are designed to only wound from the get go. but things get funny at 3100ish fps, because the the 5.56 will shrapnel more effectively and consistently then a slower moving FMJ round.
so you have flat tragetory (say easily aimed), low weight(soldiers can and do carry 210 rounds now), and fairly effective given that the first two requirments are met perfectly. and dont forget soldiers come in all sizes. in ww2 there was an alarming number of soldiers that were afraid of the recoil of there 30.06 rifle. personally 60 rounds of .308 can make my shoulder sore.
but that said, our rifle isnt perfect for combat, and our ammo isnt perfect for urban warfare. a new everything would be nice for the guys.
back on topic lets see some more rounds fired at the plates! 50 cal? release the BFG! lol