7.62×39mm
It was based on the 7.62×54mmR cartridge. The bullet size is the same, but the case is much smaller so that soldiers could carry more bullets. It was made because the Soviet Union learned that full-sized bullets were not needed in most firefights. Most firefights happened in less than 300 meters, or medium- and short-range. At this range, it is more important to have many smaller bullets than a few large bullets.