Ability to shoulder, it's a stock, it's not stock is actually irrelevant absolutely because the ATF shot themselves in the foot (pun intended) on the matter. They have changed their position concerning "shouldering" a pistol brace several times. At first it was illegal to do, then it wasn't, then it was, then it became not illegal, but vague (there's a common theme here) in that if shouldered "incidentally" to the operation of the weapon then it's ok...you just aren't supposed to shoulder it all of the time. That was their last official letter on the matter.
Concerning what they're going to do if people simply don't comply with the brace ban, we can get a potential glimpse if we observe what has/is happening around the FRT's (forced reset triggers). As some may recall, a man invented the FRT and marketed it under the business name of Rare Breed (there is also the Hellfire, another FRT). The ATF sent them a cease and desist early last year and he took it to court and lost when the ATF deemed FRT's as "machine guns."
Not long after, the ATF then sent letters to everyone who had purchased an FRT notifying them of the requirement to turn them in as they were now illegal. They went through the dealer books and went after every person that purchased one.
That is one way they could go after a lot of brace owners. A bit larger mountain of paperwork and purchase records to sift through, but never under estimate the government's tenacity in tedium if they're on a mission to trample your rights. If you ever purchased a pistol brace(s), it's possible you might receive a letter from the ATF, or possibly even an actual visit.
To further muddy the already opaque waters, one of the options concerning the braces was to remove AND RENDER UNUSABLE the brace. The ATF, after yet another swirl of mud, removed that verbiage, but then once again added it. Additionally, the ATF stated that possession of a functional brace and a pistol would constitute "constructive possession."...and all of this right as Dettelbach testified under oath in Congress that all one has to do is remove the brace from the pistol and all is legal. He was even asked spoecifically if it had to be destryed, he said no. He was asked if they would prosecute anyone for constructive possession and he said no, the parts just had to be separated ...in complete conflict with what the ATF had just released. So, right now, no one knows what the heck the ATF intends (but we can certainly guess). I can see a "I misspoke" coming down the path from their own director.
From the ATF's website as of this posting (Factoring Criteria for Firearms with Attached “Stabilizing Braces” | Bureau of Alcohol, Tobacco, Firearms and Explosives
"Other compliance options provided under the final rule are the following:
- Remove the short barrel and attach a 16-inch or longer rifled barrel to the firearm,
- Permanently remove and dispose of, or alter, the “stabilizing brace” such that it cannot be
- Turn the firearm into your local ATF office,
- Destroy the firearm
Yet, the director of the ATF said this 2 weeks ago: