I've never fired one, but I've seen them fired. Really kind of big and clunky, and when it comes down to it, not really useful in fully automatic. 300 RPM is also not much faster than you can fire a semi automatic. Some actions even allow semi auto faster than this.
For the same amount of firepower with a gun you can get now over the counter, I suggest a Saiga 12 converted to conform with the dumb ass sporting purpose or US made ban I believe it was Bush sr signed.
With the overall shape and action of a regular AK, they are just about the nicest defense shotgun available. If you use 20 or 30 round drums, and put a high efficiency muzzle brake or a shortened gas system and a suppressor on a Saiga, you will have a lightly recoiling weapon of among the highest devastation of firepower available in a shoulder fired small arm. About on par with LMG devastation out to 100 yards or so when using buckshot.