RE: whats the difference between .308 and a 7.62mm X 39mm, or are they the same?
No, these two rounds are in NO WAY the same, nor interchangeable. The 7.62x39mm is a soviet cartridge that fires a 0.311" bullet. The .308Winchester is interchangeable with the 7.62NATO, which are essentially identical in specs, and is an american derivative of the .30-06 based around a .308" bullet...
There are a great deal of "7.62mm" rounds out there...but it's kinda like the 7mm's, there's 7mmRem Mag, 7mmWSM, 7mm SAUM, 7mm-08, 7-30 Waters, 7mm STW, and so on and so on, all have kinda the same first name, but they're VERY different rounds.
Kinda like how guys named Richard, I've known probably thirty different Richards, the only way to tell them apart in a phonebook is their LAST NAME.