Some would argue but 7.62 x 51 and .308 are the same. At least my old L1A1 will use both.
Functionally they are the same thing, there are some differences in technical pressure specs (and therefore probably powders) and tolerances, but in a sound weapon they shouldn't matter.
Similar with 5.56 vs .223, except that while .223s will handle XM193 ammo just fine, the newer military ammo (XM855 or anything else loaded with the NATO SS109 spec bullet) will fire but won't hit anything since spec .223 commercial rifling doesn't spin it fast enough make it fly straight or keep it stable...it will turn a minute-of-angle rifle into a degree of arc rifle, and the bullet will likely arrive sideways.