Some sources claim the weapon is an XM214 Gatling gun, part of the "Six-Pak" system which fires the smaller 5.56x45mm 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......"