7.62x39mm
A Soviet intermediate rifle cartridge inspired by America's .30 Carbine and Nazi Germany's 7.92x33mm Kurz cartridges and designed for the SKS and RPD. It was later used in the AK-47 (and all the clones of it made by every communist country ever), AKM, RPK, Vz.58, Rk62, Rk95, Valmet M78, Galil, Ruger Mini-30, AK-103 and 104, and AK-12, among others (every 2nd World country and many 3rd World countries adopted it as their main service rifle round because the Soviets told them to, and many civilians love to buy semi-auto surplus or civilian-legal original versions of the guns mentioned above). It is comparable in power to the .30-30 Winchester and is therefore an excellent round for hunting deer. It is analogous to the 7.62x51mm NATO/.308 Winchester, but much less powerful. The top 3 users of 7.62x39mm are (former) Communists, terrorists, and right-wing Americans (simply because they are way too obsessed with the movie Red Dawn).
The USSR officially replaced the 7.62x39mm cartridge with the 5.45x39mm in 1974, but it is still used by urban Russian police forces and sometimes used for special operations (with the AKM or AK103/AK104, NEVER with the AK-47 anymore) because it penetrates light cover better than 5.45 (at the cost of lower accuracy and range, however).
The USSR officially replaced the 7.62x39mm cartridge with the 5.45x39mm in 1974, but it is still used by urban Russian police forces and sometimes used for special operations (with the AKM or AK103/AK104, NEVER with the AK-47 anymore) because it penetrates light cover better than 5.45 (at the cost of lower accuracy and range, however).
Most people believe the 7.62x39mm is most effective when fired into the air wildly while screaming, but this isn't necessarily the case.