7.62×39mm

The 7.62×39mm round is a rimless bottlenecked intermediate cartridge of Soviet origin that was designed during World War II. Due to the worldwide proliferation of the Soviet SKS and AK-47 pattern rifles, as well as RPD and RPK light machine guns, the cartridge is used by both militaries and civilians alike.Wikipedia
 
Bullet diameter7.85-7.9 mm (0.309-0.311") SAAMI 7.92 mm (0.312") CIP
Filling weight1.605 – 1.63 g
Rifling twist240 mm (1 in 9.45 in)
Case length38.70 mm (1.524 in)
Case capacity2.31 cm3 (35.6 gr H2O)
Rim diameter11.35 mm (0.447 in)
Base diameter11.35 mm (0.447 in)