The Combat MG is modelled on the M249 SAW, a belt-fed light machine gun built to lay down sustained fire. It carries a deep magazine and keeps shooting long after a rifle would run dry, at the cost of weight and a slower handling feel. This is a weapon for holding ground and suppressing a crowd.
A heavier gun like this may sit behind progression at a gun store in the Ammu-Nation mold, or turn up during bigger missions. GTA 6 gun shops are not detailed by Rockstar yet, so the exact unlock is unconfirmed.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.