The PK is a 7.62 mm

7.62 mm caliber

7.62 mm caliber is a nominal caliber used for a number of different cartridges. Historically, this class of cartridge was commonly known as .30 caliber, the Imperial unit equivalent, and was most commonly used for indicating a class of full power military main battle rifle cartridges...

 general-purpose machine gun designed in the Soviet Union

Soviet Union

The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 and currently in production in Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

. The PK machine gun was introduced in the 1960s and replaced the SGM and RPD

RPD

The RPD is a 7.62mm light machine gun developed in the Soviet Union by Vasily Degtyaryov for the intermediate 7.62x39mm M43 cartridge. It was created as a replacement for the DP machine gun chambered for the 7.62x54mmR Mosin rifle round...

 machine guns in Soviet service. It remains in use as a front-line infantry and vehicle-mounted weapon with Russia's armed forces, and has been exported extensively.
The original PK (Пулемёт Калашникова: Pulemyot Kalashnikova, or "Kalashnikov's Machinegun") was a development of Kalashnikov

Mikhail Kalashnikov

Lieutenant General Mikhail Timofeyevich Kalashnikov is a Russian small arms designer, most famous for designing the AK-47 assault rifle, the AKM and the AK-74.-Early life:...

's automatic rifle design, firing the 7.62x54mmR Eastern Bloc standard ammunition originally from the Mosin–Nagant.