Developed and introduced for military use by Russia in 1943, the 7.62 x 39mm is the world's most popular assault rifle cartridge. Thirty to fifty million Kalashnikov rifles have been made in this caliber in about a dozen countries, and close to 55 minor and major nations use it. Even though Russia has now adopted its version of the U.S. 5.56 x 45mm called the 5.45 x 39mm, it will be a long time before we see the last of the 7.62 x 39mm. The 7.62 x 39mm is also the parent cartridge of the .220 Russian, .22 PPC, and 6mm PPCcartridges. Some say the 7.62 x 39mm is equal in power to the .30-30 Winchesterbut this is not quite true. Although heavier bullets can be loaded in this cartridge, velocity is quite low since it was designed to push bullets from 120 to 125 grains to just over 2300 fps. In other words, no 7.62 x 39mm load can equal the .30-30when its loaded with a 170 grain bullet. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.