A
ballistic round,
bullet, or cartridge.. to be more precise:
Common
Russian Service Round.
The name 7.62x39 comes from the dimensions of the round.
7.62 mm in diameter, and 39 mm long.
Some of the current rifles that use this round include the
SKS and the
Mini Thirty.
Expect to pay about $5 US for 50 of these bad boys.