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". Shooting oversize bullets could raise pressures, while shooting undersize bullets might produce poor accuracy.
To remain safe and simplify loading, I shot Nosler 0.308"-diameter Ballistic Tips in a CZ 527 Carbine chambered in 7.62x39 mm. Bullets were seated with Hornady Custom dies, which come with one expander ball to size case necks to hold 0.311"- and 0.312"-diameter bullets and a second for 0.308" bullets. Accuracy was actually slightly better than for 0.310"-diameter bullets.
Relatively fast-burning spherical propellants pack tightly into the 7.62x39 mm’s small case to produce the highest velocities at the cartridge’s mild 45,000 p.s.i. maximum average pressure. Hodgdon’s CFE BLK, developed expressly for the .300 Blackout, worked well, sending the Ballistic Tips from the CZ’s 18.5" barrel at 2491 f.p.s. It also dispensed precisely from a measure directly into cases, resulting in an extreme spread of velocity of 27 f.p.s. for 10 shots.