I was at the range yesterday. There was a guy there shooting a AR-15 pistol he had built with a Sig brace on it. He was using it like a shoulder stock. We got to talking and I looked at it. This thing is a shoulder stock, period. It's got some kind of strap on it, but I couldn't see how you could even get your arm through the small opening that is supposed to accommodate your wrist, (forearm). Perhaps if you were 8 years old it could be used as it is supposedly designed. There was no way I could do it. It would have been much too tight.... Almost like a tourniquet.
This is another one of those gimmick accessories that get around a already existing ATF law. My guess is these things will be banned in time. They do make an AR-15 pistol look cool as hell instead of having that big, foam covered tube sticking out the back. If I remember correctly, there was some kind of trigger that allowed full auto type firing of a semi auto that met the same fate. It was released for sale to the public with an, "ATF Letter Of Approval", only to be rescinded by the ATF a short time later. The people who bought them took the gas pipe on them. If you're in possession of one today on a gun and get caught, it's Crowbar Hilton time.