ATF released a letter tightening the use of SigBrace. According to the new letter
-Using SigBrace as stock is 'redesigning' of the gun as it changes function of the item
-That means to use SigBrace as a stock requires ATF Form 1 and pay applicable tax.
Past letters did not go in to details on how SigBrace can be used but this letter pretty much clamps down on any use other than as an arm brace(as the product was intended)