From a glance it looks like the more common 7.62x39mm, but this is a rather obscure and now obsolete caliber from Czechoslovakia. It was used in the Vz. 52 rifle, a Czech variation of the SKS. Oddly enough this caliber has higher velocity and muzzle energy than the 7.62x39mm but was phased out to conform to Com-Bloc standards. The Czechs even rechambered some Vz.52 rifles to use the more widely adopted 7.62x39mm.