To my knowledge, most load manuals call for a .310" diameter bullet for the 7.62x39mm. I have read that some older Mini-Thirty's have a .308" bore, but I'm not that familiar with all of the details.
Hornady Third lists .308" diameter bullets for 7.62x54mm, but that is an oddity or mistake, IMO.
About all Soviet bloc 7.62mm ammo, that I'm aware of, uses the .310/,311" bullets.
If slightly undersized bullets work okay, I don't think there's any major issue about using them. Heck, I run .355/.356" bullets regularly in my GP100 (.38 Spl./.357 Mag.); no issues whatsoever.