To my uneducated ear it sure sounded like a belt fed weapon. I wonder if a bump stock could work with a belt.
I don’t see why not, but there wouldn’t be any need since belt fed weapons are almost always full auto anyway. In any event it seems to work with a bipod.
If you study it you’ll find that the RPM can vary because of how you hold it with the bump stocks. If you let your shoulder or body deflect to absorb some recoil it slows down because it’s not going into the stock. Pure Physics.