The Colt Commando is one of numerous variants of the Colt Automatic Rifle-15 (CAR-15). It is very similar to the M16A2 assault rifle and the M4 Carbine, but utilizes a scaled down barrel, buttstock, and its overall length is greatly reduced. Colt actually classifies it as a submachine gun.

The Commando is a special variant of the CAR-15, which is essentially the spiritual forerunner to the M4. It's actually in use with some USAF pilots still. Search by the name Colt Commando or CAR-15, I believe its based on that assault rifle. The closest gun I found was the SIG SG 550.

Valmet M78/83 This is the model used in the film. The Valmet M78 series of light machine guns came in 7.62x51mm (. 308) and 7.62x39mm calibers. Matrix fires the Valmet M78/83.