The Vulcan minigun is an updated version of the old Gatling Gun (famous from many old western movies).
Rather than having a someone crank the barrels round by hand (in the manner of an organ grinder)the minigun has an electric motor which gives the weapon an astonishing rate of fire (5000-6000 rounds per minute)
Now manufactured by Dillon Aero as the MK44 Minigun, It is now used by the Royal Navy for protection against 'speedboat suicide bombers' etc.
No matter how cool it looks in Predator you probably shouldn't attempt to fire this weapon from the hip. In fact it's probably best not to use it at all while dismounted, unless you can find someone really strong and gullible to carry your ammo.