Hollywood prop gun, built on an M2 browning receiver. Made to resemble a Soviet machine gun.
In some of the Rambo movies they did this, and some even used M60 recievers, mocked up to resemble DshK guns.
They did it, simply because the correct real guns were not available. Remember, its a movie, and the gun doesn't have to be real, it just has to look real enough for a few seconds at a time.