the only way a sig brace will ever be illegal to fire from the shoulder would be if they make the sig brace itself illegal. The laws on firearms are based on what that object was designed for, not how someone "may" use it. If i take my hammerless revolver, and put it on my shoulder and shoot it, it doesnt magically become illegal. its still a pistol. I can shoot my 1911 left handed while hanging upside down from a tree wearing pink lipstick on my forehead. doesnt change what the 1911 is. you shoot a rifle from your hip, not illegal. You put a shoulder stock on a pistol, thats illegal. You put a sig brace on an ar pistol, not illegal. you put that brace on your forehead to shoot, its still the same thing, an arm brace.