Similar to how the RPK is the light machine gun version of the AK-47, the M78 is the light machine gun version of the Finnish Rk 62 assault rifle, which is a slightly altered version of a Soviet AK. It shares many of the design features of the Rk 62, such as its distinctive slanted upwards milled receiver for easier reloading, a feature which later inspired the Israeli Galil assault rifle. The M78 has the standard "club-foot" RPK-style buttstock and either a wooden or black Synthetic forearm. The M78 is sometimes is used to stand in for the RPK in some films. The main differences between the RPK and the M78/83 are a different muzzle brake, front sight assembly, and many other structural differences. Aside from its flagship caliber of 7.62x39mm, the M78 has been chambered in 5.56x45mm NATO (.223 Remington) and 7.62x51mm NATO (.308 Winchester).
In the early 1980s, Valmet released a version of the M78 with a black Synthetic Thumbhole stock and black Synthetic Foregrip, called the M78/83. The later Valmet M78/83 has a synthetic thumbhole "Dragunov Style" stock. This is the version used in the film, as well as the 1985 Schwarzenegger film "Commando", which was very similar to Rambo.