I have a shockwave brace on an AR pistol, it is less of a stock than a sig brace is. The only difference is on you put your arm in the other, you strap to the side of your arm. The ATF has ruled that both are legal, to include another type that is adjustable.Well they have said you can shoulder it and then also said you cannot. The real grey area is when you look into some of the things like the shockwave "stabilizer", how is that not considered a stock.
This letter sort of makes it black and white, not gray.