Most 7.62x39 milsurp bullets mike around .309, commercial anywhere from .308 to .311. Naturally, .308 bullets are most accurate in this barrel.
The key is using a properly throated reamer for a .308 bore. It has a longer freebore. Bill Ruger figured that out when he first designed the M77 MkII and Mini 30's with .308 barrels. The surplus bullets just leave the case before engaging the rifling.
Fellow CGN'r Ganderite used a .307 bore for his surplus testing. Take a look at his sticky thread in the Red Rifle section.