i know it is metric, because of n.a.t.o.. i will concur that the brass on military cases is of different thickness. as far as the chambers being different, every one of my reloading manuals state that the 223 is the same as the 5.56 nato; except for the 62 grain bullet.
until someone can prove that there is an actual physical difference, i say nay. if there were, they would have to be different cartridges. maybe the throats are cut a bit deeper on the 5.56 to allow the longer bullets, but that is a rifle difference (same as twist rate), not a cartridge difference. if ANYONE can prove me wrong, i welcome it (then i can say i have learned something); but i doubt it can be done.