A new letter issued by the ATF to a custom shotgun shop could signal a change in the way the SIG Brace is being treated by the ATF. According to the letter, addressed to Black Aces Tactical, a custom shotgun shop, it appears to call into question the act of shouldering a firearm equipped with the brace.
That ATF letter of approval can be replaced with a new letter changing the legal standing at any time. A friend had a Ingles Hi-Power with a slotted grip and a repo stock that was legal without any restrictions before a new letter made the combination a regulated item! Never take the strap off the Sig brace it could be looked at a stock at that point.
it appears to call into question the act of shouldering a firearm equipped with the brace.