7.62.39?? man thats nothing
.303 British, or 7.7mmx56R, is a .311 inch calibre rifle and machine gun cartridge first developed in Britain in the 1880s as a blackpowder round, later adapted to use cordite and then smokeless powder propellant. It was the standard British and Commonwealth military cartridge from 1889 until the 1950s when it was replaced by the 7.62x51mm NATO.
The measurement .303 inches (7.7 mm) measures the nominal size of the bore measured between the lands which follows the older blackpowder practice. The overall size of the bore which is .311 inches nominal though bores for many .303 surplus military rifles often can be found ranging from around .309 inch up to .318 inch.
This cartridge saw much sporting use with surplus military rifles, especially in Australia and Canada, and to a lesser extent, in the United States. In Canada, it was found to be adequate for any game except the great bears. In Australia, it was common for old military rifles to be re-barreled in .303/25 and .303/22.
not only did it kill many men but it also killed man deer pigs goats etc for deer over a million was shot with the 303 and it is still used to this day for hunting, even the elephant was shot with the .303 british round.
as for the round you stated
The Soviet 7.62x39mm rifle cartridge was designed during World War II and first used in the SKS carbine. The cartridge was likely influenced by a variety of foreign developments, especially the pre-war German GeCo, 7.75x39mm experimental round,[3] and possibly by the late-war German 7.92x33mm Kurz ("Kurz" meaning "short" in German). Shortly after the war, the world's most recognized assault rifle was designed for this cartridge: the AK-47. The cartridge remained the Soviet standard until the 1970s, and is still one of the most common intermediate rifle cartridges used around the world. Its replacement, the 5.45x39mm cartridge, has less stopping power and armor penetration, but is highly lethal, has a flatter trajectory, and is more controllable in fully automatic fire due to the lower recoil. The change was in part a response to NATO switching from the 7.62x51mm NATO cartridge to 5.56x45mm NATO
the .303 was way invented before that dude. no other round can say they have killed so many humans and animals like the .303 can, and i still hunt to this day with the 303 its dependable and gets the meat home