From another forum:

Well,

Having beat this to death, and my exhaustion, I finally have a feel for the situation. My original dilemma was "can I put a 308 bore on a 7.62 X 39" gun. Lots of good people have said "NO" because the 7.62 X 39 bullet is "oversize" and would be dangerous. That is certainly good solid advice. Others have said that 309 or 310 should be OK and that is probably true, as well. So who will I kill? Turns out this is a really deep question with some surprising answers. Follow this:

The 7.62 X 39 round is only "sorta" correctly dimensioned. ACTUALLY the round is ....308( .5 or 9) at the leading part/front of the bullet after the Ogive. In the final 5 or 10 thousandths of the bullet the diameter swells to .311. That is like putting a gas check on the rear. With that design it would perform, ballistically, like a full .311 bullet in a .312 or 311 bore. It would also fire safely in a .308 bore. Recently learned that Savage made a 7.62 X 39 that had exactly a 308 bore. No alarms going off in the community about the Savage bolt guns being lethal to the shooter. Also, no "WARNING odd size bore" stamped in the barrel. Others have done this exact same thing with other firearms, including the VZ 58 foundation design and all has worked out swimmingly. The VZ 2008 had a 308 bore and the other Century VZ 58 clones, using us made barrels, had 308 bores. What a cluster FXXX, right? Conundrum...at least...right. The design of the Russian 7.62 X 39 round is the answer as it is 308 "mostly".

Here is the real danger. If I thought that .311 or .312 Russian bullet was actually that size....I might be tempted to HAND LOAD .312 bullets and use them in a 7.62 X 39 chamber. And that might simply explode in my face.

Thank this guy:

Minesweeper3433

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Bullit OVERSIZE
« Reply #21 on: Yesterday at 11:22 PM »

Quote

Built a 7.62x39 bolt gun on an mini mauser for shooting surplus ammo out of using a 308 bore. Went with 308 as on measuring rounds we pulled found that only the last 5-8 thousandths were larger than 308. Almost like a skirt. That gun shoots great to this day with steel cased ammo or whatever else we feel like putting in it. We put a long 1 degree throat in it as well when ordering the reamer. Although I was always taught that the AK series of rifles had a long throat for that reason. This was however from an instructor at Aberdeen that I didn't hold in to high regard. Will pull a few soviet rounds over seas here today if I can get my hands on a few.

Old guns like old friendships are long retained and must be constantly renewed. James V. Howe The modern gunsmith Pub. 1934 People where smarter before I was born.

Navy...might have known...right?

Good luck,

John