I was at an event called Kalashbash here in TX a few months ago. One of the guys at the range, we'll call him "The Tinker" (if you're a Louis L'Amour fan you'll know what series of books that name comes from), had a .22LR Gatling Gun, kind of like (or maybe even)
this. Only he wasn't satisfied with the rate of fire that it had so he put a gear box and servo motor on it (I think that's what was in the box attached to the gun). He had a shiny chrome pushbutton to fire it. It wasn't loud of course but it was very, very fast. Like mini-gun fast.