Here som info on the gun from IMFDB:
Nicknamed "Old Painless", a hand-held M134 Minigun is the main weapon carried by Blain (Jesse Ventura). It has been modded for handheld use with an M60 handguard assembly (installed backwards) and a rear pistol grip which is taken from normal (at the time) Minigun spade grips. The grips are attached to the weapon by a custom Y-frame with an M16-style carry handle that mounts to the weapon's recoil absorbers. The weapon was powered by an electric cable hidden off camera and fired blank rounds to ease the recoil force; in addition, the rate of fire is substantially decreased from the normal 6,000 RPM to 1,250 RPM. There are several reasons for this; to ease recoil, save on ammunition, and because director John McTiernan wanted the barrels to be visibly turning rather than a blur. It is believed in real life a similar weapon was tested by US special forces in the 1970s but found to be impractical.
Some sources claim the weapon is an XM214 Gatling gun, part of the "Six-Pak" system which fires the smaller 5.56mm NATO rounds. The XM214 never made it beyond testing and the film weapon is clearly the larger M134 as evidenced by the non-tapered barrels, and the four-disc barrel clamp. On the subject of the weapon's caliber the film's technical adviser Kevin Dockery had the following to say:
"The Predator gun is an M-134. It was never a Microgun (XM-214). That story has been rattling around the Internet and elsewhere for years, that the Predator gun was a 5.56, it wasn't. Ventura had a hand in putting together the harness that held the gun, after all, he had to carry and handle the darn thing, and told me a bunch about it. When Dan had the gun (past tense I believe) he contacted me to see if Ventura wanted to buy the weapon. I'm going to do a writeup on the XM-214 including pictures of me holding one. It isn't a big deal to pick the microgun up and hold it cradled in one arm. Maybe when people see just how small the 5.56mm gun is, this story will start going away. The pack in the movie held all of four seconds worth of ammo and no batteries. In the first scene when Ventura fires the gun, you can see the cable for it in the dirt behind him. And the trigger didn't work. The special effects man handled powering the gun for several reasons, including safety. Something about them not wanting the actors injured if the gun was dropped and the trigger pulled."
The gun is also operated in the film by Mac shortly after Blain's death and Mac uses it to chop down half the forest in an attempt to kill the Predator. The ammo pack for the weapon uses an M23 armament system ammo box and cover assembly attached to an ALICE pack frame. These ammunition canisters hold approximately 550 rounds when filled; at the confirmed rate of fire (1,250 RPM) when the weapon was sold, the minigun had a maximum of 25 seconds of ammo.
For the firing scenes the box is oriented correctly with the feed chute attaching to the upper left corner of the box and oriented on the weapon so the window is facing upward. During the non-firing scenes the ammo box is positioned up and down with the chute turned upside down, hiding the fact that there is no ammunition present. Also for non-firing scenes, the weapon is not hooked up to a power supply, as the pack only contained ammunition, not batteries. This allowed the actor to move freely during these scenes. During the firing scenes, the batteries were just off camera and the weapon was shot so the cables would not be visible; the cables are trailed along the ground and then routed up the actor's trouser leg and through their clothing to the gun.