5.45x39 ammo ammunition is a lightweight, high-velocity cartridge primarily used in the AK-74 rifle and its variants. Developed in the Soviet Union in the 1970s, it was designed to improve on the performance of the older 7.62x39mm cartridge. The 5.45x39mm round features a smaller, lighter projectile with a pointed tip, offering better ballistic performance and armor-piercing capabilities compared to its predecessor.