Someone mentioned it earlier; why not get a Tavor? You get the shrunken-down package and still remain legal. No NFA paperwork, simple to buy and sell, and you can use either 5.56 or, with the conversion kit, 9mm. Granted, it doesn't address the issue of the Sig brace and the underlying freedom of choice to use it or not, etc., but for those who don't want to be a test case, the Tavor is a viable alternative. Fun, too!