 The weapon that Blain (Jesse Ventura (qv)) is using is a minigun.  This is a weapon most commonly mounted on the side of a helicopter  (or an aircraft carrier) and many, many modifications had to be made  to make it usable in the film. It was powered via an electrical  cable hidden down the front of Blains trousers. The firing rate was  slowed down to approximately 1/3rd the normal rate of fire, both to  reduce consumption of blanks, and to make the spinning of the  barrels visible on film. It is rumored that Ventura had to wear a  bulletproof vest because of the forceful ejection of spent  cartridges, but this is false. Unmodified miniguns eject out of the  bottom, with the cases essentially falling out due to the force of  gravity. Close examination of the film (especially the scene in  which Mac fires the minigun at the fleeing predator, along with the  other commandos) show that the ejection of the minigun was not  changed.