The 5.45x39mm cartridge is a rimless bottle-necked rifle cartridge. It was introduced into service in 1974 by the Soviet Union for use with the new AK-74 assault rifle. It gradually supplemented then largely replaced the Soviet 7.62x39mm round in service.
The Soviet 5.45x39mm cartridge was developed in the early 1970s by a group of Soviet designers and engineers under the direction of M. Sabelnikova. Further group members were: L.I. Bulavsky, B.B. Semin, M.E. Fedorov, P.F. Sazonov, V. Volkov, V.A. Nikolev, E.E. Zimin and P.S. Korolev.
The Soviet 5.45x39mm is an example of an international tendency toward relatively small sized, light weight, high velocity military service cartridges. Cartridges like the Soviet 5.45x39mm, 5.56x45mm standard NATO round, and Chinese 5.8x42mm allow a soldier to carry more ammunition for the same weight compared to their larger and heavier predecessor cartridges and produce relatively low bolt thrust and free recoil impulse, favoring light weight arms design and automatic fire accuracy.
The Soviet original military issue 5N7 cartridge variant introduced in 1974 are loaded with full metal jacket bullets that have a somewhat complex construction. The 3.2 g boat-tail projectile has a gilding-metal-clad jacket. The unhardened steel core is covered by a thin lead coating which does not fill the entire point end, leaving a hollow cavity inside the nose. The bullet is cut to length during the manufacturing process to give the correct weight. The 5N7 uses a boat-tail design to reduce drag and there is a small lead plug crimped in place in the base of the bullet. The lead plug, in combination with the air space at the point of the bullet, has the effect of moving the bullet's center of gravity to the rear; the hollow air space also makes the bullet's point prone to deformation when the bullet strikes anything solid, inducing yaw. The primer has a copper cup and is sealed with a heavy red lacquer. The propellant charge is a ball powder with similar burning characteristics to the WC 844 powder used in 5.56x45mm standard NATO round ammunition.
Tests indicate the free recoil energy delivered by the 5.45x39mm AK-74 assault rifle is 3.39 J compared to 6.44 J delivered by the 5.56x45mm NATO in the M16 assault rifle and 7.19 J delivered in the 7.62x39mm in the AKM assault rifle.