While looking around youtube yesterday, I saw something interesting. It was a short barreled, belt fed 12 ga upper for an AR lower. somebody named Moonstalker had posted it. Looked kind of like fun.
I could not tell for sure if this was full auto. the guy was firing it off of a bench with it sitting on a bipod. It was also a very short barreled rifle type set up.1-1/4oz. at 1330 fps. on full auto in a 6lb. rifle does not look like fun.
unless you count uncontrollable sky busting as fun.
it'd be about the same fun as a full auto 45-70 with 500gr. bullets in your Winchester model 94.