Barrel bore and bullet diameters are all over the place for the 7.62x39 mm (Russian) with the former measuring from 0.308" to 0.311" and the latter being 0.308", 0.310", 0.311" or 0.312".
A Few Notes on Surplus Yugoslavian 7.62 Ammo
7.62x39mm Gel Test - 7 Loads Put to the Test - Firearms News
Best AK-47 Ammo: 7.62x39 Steel VS Brass - Wideners Shooting, Hunting & Gun Blog
Reloading the 7.62×39 mm Russian: Load development with 125 and 150 gr. bullets –
Barnaul 7.62x39 123gr FMJ Steel Cased Ammunition - 20 Round Box - Clark Armory
Handloads: .30-30 Winchester An Official Journal Of The NRA
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GUNS Magazine Reloading the 7.62x39 - GUNS Magazine
Russian 7.32 x 39 in stock $ .41ea.
Handloads: 20 Gauge An Official Journal Of The NRA
Best AK-47 Ammo for Defense and Performance - Firearms News
Handloads: .25-35 Winchester An Official Journal Of The NRA