The pistol stabilizing brace was neither “designed” nor approved to be used as a shoulder stock, and therefore use as a shoulder stock constitutes a “redesign” of the device because a possessor has changed the very function of the item. Any individual letters stating otherwise are contrary to the plain language of the NFA, misapply Federal law, and are hereby revoked.
Any person who intends to use a handgun stabilizing brace as a shoulder stock on a pistol (having a rifled barrel under 16 inches in length or a smooth bore firearm with a barrel under 18 inches in length) must first file an ATF Form 1 and pay the applicable tax because the resulting firearm will be subject to all provisions of the NFA.
What does this mean? On first reading, it seems the ATF has completely walked back the approval of the Sig brace and other subsequent versions. Hopefully, Adam Kraut at PrinceLaw.com will weigh in pretty quickly...they've been on top of the whole ATF waffling issue from the beginning.