In single player or multi-player mode, select the minigun and press [Secondary
Fire]. The minigun will "warm up" and constantly spin the barrels until press 
[Secondary Fire] again or switch to another weapon. While this feature is 
active, the minigun will fire immediately after pressing [Fire] -- it no 
longer needs to "spin up" or "spin down", and will also fire more accurately.
Using this technique will allow you to easily wipe out the Praetorian Aliens 
or Assault Predators. 
Note: In multi-player mode, only the Corporate AT officer can use the minigun
as a set weapon, and they are very rare weapons in a normal multi-player game.