RPK

The RPK (Ruchnoy Pulemyot Kalashnikova, Russian: Ручной пулемёт Калашникова, literally "hand-held machine gun of Kalashnikov") is a 7.62x39mm light machine gun. It was developed in the late 1950s and produced in 1961 to replace the RPD light machine gun in the Eastern Bloc.

Overview
During the Peace Walker Incident in 1974, Big Boss and his mercenary group, the Militaires Sans Frontieres, procured some design specs for the RPK while doing a holdup excercise. The MSF later modified the weapon to utilize a drum-based ammo magazine.

In-game weapon descriptions
"A light machine gun (squad support weapon) designed as an modified, expanded version of the RK47. It is similar in performance to the RK, but delivers greater firepower and more reliable operation at the cost of lower accuracy. It has a longer barrel than the RK, which somewhat reduces shaking during sustained fire. Think of it as a slight heavier, more precise RK. When choosing whether to carry an RK or an RPK, consider the tradeoff between ease of handling and firing accuracy and pick the best gun for your needs. (Rank 5) This model has been modified to accommodate a drum magazine, significantly boosting its ammo capcity. Hence, it does not need to be reloaded as often, giving it a tactical advantage over models that use standard magazines."

- RPK weapons description from Metal Gear Solid: Peace Walker