The 5.45×39mm cartridge is the main bullet made for short- and medium-range guns for the later Soviet and Russian militaries. It is used for guns like the AK-74, the RPK-74, the AN-94, the AK-100 series of rifles, and the AK-12. It is a Small Caliber High Velocity (SCHV) bullet, which means it is very small, but it has a high muzzle velocity and effective range.
After hearing about the success of NATO's new SCHV bullet, the 5.56×45mm NATO, in the Vietnam War, the Soviet Union believed they needed to make their own to be able to fight off NATO. They hired Mikhail Kalashnikov, the creator of the AK-47, to make an SCHV assault rifle. Kalashnikov disliked the idea because he felt the Soviet Union needed to outdo NATO weapons, not copy their ideas. Nonetheless, he made the AK-74, the first gun to fire the 5.45×39mm.