Its a General Electric XM214 Minigun usually mounted on the cobra helicopter
The gun ran off several truck batteries. the cables ran down Jesse Ventura's pants leg and was converted to take blanks
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.
from wikipedia -
The XM214 was a prototype 5.56mm rotary barreled machine gun designed to be man-portable. It was designed and built by General Electric, but it never reached mass production. Also known as the Microgun, the XM214 was a scaled-down version of the M134 "minigun", firing M193 5.56 x 45 mm ammunition.
The XM214 was first developed for aircraft applications. Later General Electric developed it into a man-portable weapon system, known as the GE Six-Pak. The complete Six-Pak system weighed 85 pounds (38.5 kg) with 1,000 rounds of ammunition, comparable in weight to some heavy machine guns. The basic gun in the Six-Pak weighed 27 pounds, or 12.2 kg. The system could be carried by a team of two soldiers and mounted either to an M122 tripod or a vehicle's pintle mount. Length overall is 104.1 cm , gun only is 68.6 cm long. Width (including ammunition case) is 44.4 cm. Sighting was usually by optical telescope.
The Six-Pak consisted of the XM214, the ammunition package, and the power module, and the ammunition module consisted of two 500 round, factory packed, and disposable cassettes mounted to a holding rack. Linked ammunition was fed through a flexible chute to the gun; when the first cassette was empty, ammunition would then feed from the second cassette, tripping a visible signal that a new cassette needed to be added to the rack. The power module contained a 24 volt nickel-cadmium battery, a 0.8 horsepower (0.60 kW) motor, and solid state electronic controls. Unless the battery were plugged into a vehicle's power supply, the battery's charge would be depleted after 3,000 rounds. The system could be broken down quickly into two portable loads of roughly 42 pounds apiece. This was accomplished by means of a quick-release fitting at that end of the belt chute fastened to the gun.
An 88 by 10 inch (224 x 25 cm.) 5.56mm Microgun Pod was also developed for external use on light aircraft and helicopters. The unit had an ammunition capacity of from 1,500 to 3,500 rounds and loaded weight of 186 to 300 pounds. The feed system was linkless, via a helical drum, with a nominal rate of fire of 6,000 rpm. However, adjustments to 1,000 or 10,000 rpm were possible, if desired. Power could be provided by a built-in battery pack, or from the aircraft itself.
Using the electronic controls, the weapon's rate of fire could be adjusted from 400 rpm all the way up to 4,000 rpm. Later editions of Jane's Infantry Weapons claimed a theoretical cyclic rate of up to 6,000 rpm. George Chinn, author of The Machine Gun Volume V, contended that the XM214 prototype had a rate of fire of up to 10,000 rpm, but the man-portable Six-Pak was limited to 4,000 rpm.
General Dynamics claimed a top ROF of 12,000 RPM and offered the weapon in their catalog as late as 2005..[1]
The electronic controls also contained a burst limiter and handled the automatic clearing of the gun after bursts.
http://en.wikipedia.org/wiki/XM214_Microgun
I believe this is the actual gun used in Predator -
http://www.motionpicturearmourer.com/minigun.htm
There is an airsoft version available for an extraordinary amount of money (about $5k ish). -
http://echo1usa.com/English/index.php?main_page=product_info&cPath=79&products_id=358
