I own one and they are legal. They are not very accurate because your finger keeps bumping the trigger and your shoulder acts like a trampoline for the butt of the gun. Nothing like full auto where most of the recoil is absorbed internally on the buffer spring.
The video of it in operation looks interesting and makes it appear capable of a modicum of accuracy. Thanks for the performance report...I think the cost of ammo would make this a prohibitively expensive toy, at least for me.
Looks like it would put a lot of stress on a weapon not built for such, any known issues(firing pin/other damage)using this on particular weapons?