RE: whats the difference between .308 and a 7.62mm X 39mm, or are they the same?
European cartridge designation makes more sense than the US designations, which don't mean anything. European designation is bullet diameter and length, expressed 7.62x39 (nominal .30 caliber bullet, case length is 39 mm). 7.62 Nato, AKA .308, is 7.62x51. the 7.62x54 noted above is actually 7.62x54R...the "R" designates a rimmed cartridge.
Just a quick look at the two cartridges in question will point out the different case sizes.
7.62x39, and 7.62x51.