I've got the perfect M/11 to try it onThey just put pressure on the back of the trigger (pushing it forward), which makes bump-firing easier, and more consistent. You'll have to adjust the wing nut to get the pressure right-- that does vary between guns. I'm not certain it would work with an AR; I didn't have one at the time. But it definitely works with an M/11. However-- you will need the fake suppressor, or some other muzzle device. Otherwise, there's no way to hold an M/11 to make it work. And you'd be a fool to try it with the strap, unless you're in need of a good nickname like "righty" or "stumpy".
Don't get too excited; you cannot hold the grip with your firing hand or it won't work. So you cannot be more than about "stump accurate" at 15 feet-- you can tear up a stump, but that's about it.
That company sells a lot of newer designs-- one/some might work better than the original. But the original is the one that caused the hoopla!