Valmet M-78 The Valmet Model 78 series of weapons were developed to fill the need for a light weight, higher firepower squad support weapon. This weapon fills this role better than the assault rifle, due to a longer barrel, bipod assembly, and heavier receiver.
Cartridge: 7.62×39mm Soviet M1943 Length: 1060 mm (41.73 in) Weight: 4.7 kg (10lb 6oz) Barrel: 480 mm (18.9 in) Rifling: 4 grooves, right hand twist Magazine capacity: 15 or 30 rounds Magazine type: detachable box Rafe of fire: 650 rds/min Sights: standard iron
The Valmet M78 is a heavy barreled light support variant of the M76 assault rifle. In addition to firing the 7.62x39 cartridge, it was also chambered in 5.56x45 and 7.62x51, variants. The M78 is Vamet's version of the RPK squad auto. 
M-78 7.62 x 39
In the movie "Red Dawn", the RPK's that the Wolverines were using were actually Valmet M78's in 7.62x39mm.
M-78 7.62 x 51
M-78 Short Version - 16" inch barrel
One point to note is the design of the stock. The above stock designs are commonly referred to as "Club Foot". The non shooting hand is used to grab underneath the stock, pulling it tighter to the shooter's shoulder. This allows greater control and accuracy. 
In production: 1978-86