And?...........
First, ATF didn't "approve" your pistol and arm brace combination unless you submitted it for determination.
Second,
Determinations aren't law or regulation and are only the opinion of ATF.
Regulations change.
Determination letters issued in the past change as ATF staff changes.
Laws change.
Just because "XXXX" is legal today doesn't mean it will be legal tomorrow, next month or five years from now.
IMO, and I wrote this the first day I laid hands on the Sig arm brace......."It's a shoulder stock". I get that it was supposedly designed by a disabled vet. That doesn't mean its legal. ATF could have inserted the warning "does not constitute the making of an NFA firearm when used as designed and intended" into that first determination letter. They didn't and they've been scrambling to put the kittens back in the bag ever since.
Couple that with arm brace manufacturers changing their design after submission means the ATF determination may change as well.
ATF was inundated with arm brace designs (which they don't regulate) asking "is this arm brace legal?" So many that ATF stopped issuing determinations unless the arm brace was submitted with a firearm attached.
Several manufacturers didn't bother submitting their design for determination at all.
Numerous manufacturers just modified their existing shoulder stock to allow it to be strapped to the forearm....and called it an arm brace.
Whether foolish, naïve or just plain stupid, ATF Technical Branch screwed this one up. It points out just how poorly written the National Firearms Act of 1934 really was. A law intended to prohibit (via an onerous tax) machine guns, silencers and originally ALL CONCEALABLE FIREARMS. That included handguns.
Just before submission to Congress, handguns were removed from the NFA '34. The Treasury Department originally determined that "concealable" meant 18" barrel and 26" overall length. (years later changed to 16" bbl length for rifles)
And thats why we are in this sad state of affairs. While we may think adding a shoulder stock to a handgun makes it LESS concealable, the NFA says its no longer a handgun (which is exemt from the NFA), but a concealable firearm (which is not exempt from the NFA).
I wonder which federal judge would view a handgun with a shoulder stock as a "handgun with a shoulder stock" and not as an SBR......because it was never a rifle to begin with. Federal definitions can change either by ATF's rulemaking authority, passage of a law or by the judiciary.