The 5.45×39mm cartridge is a rimless bottlenecked 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 7.62×39mm round in service. It was the Soviet's answer to the 5.56 cartridge, but is not a copy of the round, because 5.45 has a much larger bullet and has much more detrimental ballistics. The idea behind 5.45 was to be fast, smaller and devastating, rather than slow and big like the 7.62x39. This move was done to get more of an armor piercing effect from faster and more efficient ammunition, although both 7.62 and 5.45 are both still used today.
The 5.45×39mm cartridge was developed in the early 1970s by a group of Soviet designers and engineers under the direction of M. Sabelnikov. Further group members were: L. I. Bulavsky, B. B. Semin, M. E. Fedorov, P. F. Sazonov, V. Volkov, V. A. Nikolaev, E. E. Zimin and P. S. Korolev. The 5.45×39mm is an example of an international tendency towards relatively small sized, light weight, high velocity military service cartridges. Cartridges like the 5.45×39mm, 5.56×45mm NATO and Chinese 5.8×42mm 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, favouring light weight arms design and automatic fire accuracy. The Soviet original military issue 7N6 cartridge variant introduced in 1974 are loaded with full metal jacket bullets that have a somewhat complex construction. The 3.43 g (52.9 gr) boat-tail projectile has a gilding-metal-clad jacket. The unhardened 1.43 g (22.1 gr) steel (steel 10) rod penetrator core is covered by a thin lead inlay 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 7N6 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 brown-lacquered steel case is Berdan primed. Its 39.37 mm (1.55 in) length makes it slightly longer than the 7.62×39mm case which measures exactly 38.60 mm (1.52 in). 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 5.56×45mm NATO ammunition. The 7N6 cartridge weight is 10.75 g (165.9 gr). The 7N6 has been known to take off limbs due to its dangerous tumbling and is classified as armor-piercing ammunition, and as such, is illegal to import into the United States.
Tests indicate the free recoil energy delivered by the 5.45×39 mm AK-74 assault rifle is 3.39 J (2.50 ft·lb), compared to 6.44 J (4.75 ft·lb) delivered by the 5.56×45mm NATO in the M16 assault rifle and 7.19 J (5.30 ft·lb) delivered by the 7.62×39mm in the AKM assault rifle. Military 5.45×39mm ammunition was produced in the former Soviet Union, GDR and Yugoslavia, and is produced in Bulgaria, Poland and Romania. In the former Soviet Union this ammunition is produced in Russia, Kyrgyzstan, Uzbekistan and Ukraine. Early ballistics tests demonstrated a pronounced tumbling effect with high speed cameras. Some Western authorities believed this bullet was designed to tumble in flesh to increase wounding potential. At the time, it was believed that yawing and cavitation of projectiles were primarily responsible for tissue damage. Martin Fackler conducted a study with an AK-74 assault rifle using live pigs and ballistic gelatin; "The result of our preset test indicate that the AK-74 bullet acts in the manner expected of a full-metal-cased military ammunition - it does not deform or fragment when striking soft tissues". Most organs and tissue were too flexible to be severely damaged by the temporary cavity effect caused by yaw and cavitation of a projectile. With the 5.45 mm bullet, tumbling produced a temporary cavity twice, at depths of 100 and 350 mm. This is similar to (but more rapid than) modern 7.62×39mm ammunition and to (non-fragmenting) 5.56 mm ammunition.
The 5.45×39mm was developed by the Soviet Union for military use and it was not intended to create civilian weapons in this chambering. Only a few civilian 5.45×39mm weapons were developed and commercially offered. Non AK-74 platform rifles and commercial offerings include the East German Ssg 82 bolt action rifle and the Russian CRS-98 "Vepr-5, 45" semi-automatic carbine and Saiga semi-automatic rifle. In May 2008 the Smith & Wesson M&P15R was introduced. This was a standard AR-15 platform rifle chambered for the 5.45×39mm cartridge and was Smith and Wesson's first AR-variant rifle in a chambering other than 5.56×45mm NATO and is no longer in current (2012) production. The civilian version of the Israel Weapon Industries Tavor rifle for the US market includes an optional 5.45×39mm conversion kit. The US ammunition manufacturer Hornady produces commercial polymer-coated steel case 5.45×39mm ammunition loaded with 3.89 g (60.0 gr) polymer tipped V-MAX bullets with a stated ballistic coefficient (G1 BC) of 0.285. WOLF Performance Ammunition offers several Berdan primed commercial 5.45×39mm loads. The Russian ammunition manufacturer Barnaul Cartridge Plant also offers several Berdan primed commercial sporting and hunting 5.45×39mm cartridges. Barnaul states that their 5.45×39mm cartridges produce a maximal pressure of 294,2 MPa (41,054 psi) and have a bullet dispersion R100 of 25 mm (1.0 in) at a range of 100 m (109 yd), meaning every shot of a shot group will be within a circle of the mentioned diameter at 100 m (109 yd). The American firearms corporation Century International Arms offers Ukrainian made 5.45×39mm cartridges with steel casings and bi-metal (copper/steel) jacketed bullets under the Red Army Standard ammunition brand.