The gun control units are set at the factory at 3,200 rpm. When literature says variable, that is not functionally true. There is a model out now with a selector for low and hi rate, but as far as an air weapon that is a solution searching for a problem. 3,200 rpm is a good sweet spot for target saturation and sustained firing.
I was the armourer and the master gunner for the helo squadron. I fired over 100,000 rds, and some of those in combat. Not sure why ppl started calling weapons and firearms "platforms". The aircraft was the platform we mounted this weapon on.
The M134D was pretty reliable providing good maintenance and proper assembly. It liked to be greased liked to be used, so we did. It had a large hopper, which when fully loaded was very heavy - like don't bring pax heavy. The gun and mount were over 100lbs. It was very finnicky about how the hopper was loaded. #### up packing it and no brrrt. Most ammo was good, but belts are heavy, so some types of links would stretch and break. The gun pulls the belt into the feeder-delinker and another drive motor pushes ammo from the hopper. CAF ammo was top notch.
The beaten zone was tighter than you would expect and first round hits were no issue out to 1000 m. It was a lot more effective that a lot of other air weapons, like forward mounted .50 cal. We could clean out a grape drying hut with sheer volume while 25 mm could not.
As I said, it was pretty reliable, but if you did get a stoppage it was much harder to clear than a conventional GPMG. Feeder-delinker jams required opening everything up and cutting bent rounds out with diagonal cutters and a screwdriver which is a little harder than it sounds when you are chopping holes in the sky and hanging out an open door. If the belt broke it was a pain to get it re-fed into the feed chute.
It generated 400 lbs of thrust and you could feel the muzzle blast thru my plates. Also I have some signifigant hearing loss. But dumping a thousand round burst was just as cool as you might imagine, and watching them fire at night at ground targets was one of the coolest things I saw.