The RPK ("Ruchnoi Pulemyot Kalashnikova") entered service in 1961 as a powerful firearm, developed from the PK series of weapons. Externally, the weapon is essentially similar in appearance to the AK-47 assault rifle. This similarity is due to both systems being developed by the prolific and highly successful designer Kalashnikov, the hero of the Soviet Union. The success of the AK-47 led to the Soviet military asking for a similar design to replace the light machine gun that was in service at the time - the Degtyarev RPD. So Kalashnikov lengthened the AK with a heaver barrel and fitted a bipod to support sustained automatic fire. Kalashnikov also stole the RPD stock design, which was popular with the troops, and changed the rear sight to allow for windage adjustments. The 30-round AK magazines were interchangeable, however, larger magazines of 40 rounds and a 75-round drum were needed to satisfy the full automatic requirement. The gun was eventually ordered for the Red Army in 1961 but did not enter service until 1964. Each infantry squad was issued one RPK along with the 75-round drum magazine. Airborne units were issued the RPKS featuring the folding stock.
The RPK was produced to operate with ease and to be nearly jam free. A major bonus was the commonality of parts with the AK service rifle series. The RPK utilizes a simple gas operated system firing from a closed bolt. Pulling the trigger begins the firing action - a round is fired and gas is produced when the powder burns and enters a small gas port pushing small pistons. The bolt is pushed back and the gas pressure is released allowing the bolt to be pushed forward by a spring. For night actions the NSP-2 infrared sight can be attached.
As with all weapons the RPK has her drawbacks. The closed bolt firing action and the barrel are not a quick change in the field. If the operator has good fire discipline few problems occur, however in combat situations discipline can go out the window. Optimal sustained fire is around 80 rounds per minute - if overused, chambered rounds could "cook off" and barrel damage could ensue. The size and weight of the box magazine is limited the amount of ammunition carried by the weapons crew - very different from the amount of belt ammunition carried around the necks of US soldiers in Viet Nam for the M60 general purpose machine gun.
The RPK is still in service, seeing first combat action in the Vietnam War while it is being used by terrorists today in Afghanistan and around the world with many thousands having been produced.