The 5.45x39mm (5.45 Russian, 5.45) round was introduced into service in 1974 for use with the new AK-74 assault rifle. It gradually supplemented and then largely replaced the 7.62x39mm round in service with the Russian military, and is now the current standard service ammunition for the Russian military.
The 5.45x39mm is a rimless bottleneck cartridge, and virtually all of the military 5.45 produced is steel-cased. Most 5.45x39mm cartridges are Berdan primed; some are Boxer primed, and use small rifle primers.
The bullet diameter is nominally 5.45mm; the actual diameter is 5.62mm. It fires with a maximum pressure of about 55,000 PSI, and is best fired in a barrel with a 1:10 rifling twist; if the barrel is shorter, the twist may be considerably tighter to help stabilize the bullet in flight.
There are a few variants of 5.45x39mm, and it has a growing commercial market; new developments are very recent.
This cartridge has a red ring above the cartridge neck to identify it. It has a hardened steel core (the steel rod penetrator), and was developed in response to increased body armor use on the battlefield. The bullet weighs in at 52 grains.
There are several commercial 5.45x39mm loads. Some are JHP, and are usually marketed under the Wolf brand, though Tula (Tulammo) produces some 60 grain bullets. Hornady produces a 5.45x39mm load with a ballistic tip.