I'd guess some barrels are available in 7.62x39 with a standard .308 bore, some with the correct .310 bore. The x39 normally uses a .310 barrel, Aks, SKS, Mosins, Enfields and Arisakas all are .310, not .308 like Amercian, Swiss or French weapons use. I believe Ruger Mini 30s had this in there early variants, later ones had correct bore size. .002 aint much, but might add some extra pressure swagging a .310 bullet into a .308 bore. The advantage to the .308 barrels, alot more choices in reloading and rolling your own for accuracy, .310 advantage use any normal commercial ammo without worry.