In the part where Blain has just died, Mac starts shooting with his rifle at the Predator. He then picks up Blain's minigun and starts shooting. Everyone joins in. When everyone has stopped shooting, Mac is still pressing the minigun trigger, but it's not firing anything. Presumably, that means he's run out of ammo. But when the camera cuts to another angle, you can see lots of unexpended bullets hanging out of the gun.