So, your options are:
A: Register it as a Short Barreled Rifle, under amnesty, for free, and get an NFA stamped form. DOWNSIDE: You must file a Form 20 for permission every time you want to take it out of state. NOTE: No, sticking a rifle barrel on doesn’t make it not an NFA weapon. Once it is on the Registry, it remains an SBR, no matter what barrel you put on it, unless you ask them to remove it from the Registry, and either destroy it or make it not an SBR.
2) You can remove the brace. The brace is perfectly legal on any rifle it fits. If you own such a rifle, you now have a rifle accessory, and a pistol with no brace. You may eventually be able to put it back on, if the courts do their job and tell ATF to cut the crap. Or, you can apply for a stamp later if you wish. It will cost $200. It might be worth the wait.
c] You can leave it in illegal format. I recommend against this. I especially recommend against beating your, um, chest on social media that “I WILL NOT COMPLY!” Unless you really hate your dog and want ATF to shoot him when they come to arrest you.
IV} You can destroy the brace (if you’re an idiot) or surrender it to ATF (if you’re a bigger idiot).
E. You can destroy the weapon (if you’re a moron) or surrender it to ATF (if you’re a complete retard).
What To Do About The ATF’s Pistol Brace “Interpretation” (michaelzwilliamson.com)
Go read the whole thing.
Just take it off and put it away… or SBR it and put a REAL stock on it..
The problem with taking it off and storing it with your guns, or even in your house/place of business, is the “can be converted” part of it all. (Paraphrased, as I do not have the actual text of the rule handy.) Basically, if you have the pistol that used to have the brace on it, and the brace itself, the ATF will consider that a fully assembled SBR. You need to have the brace somewhere else, in my (IANAL) opinion. A storage bin in your crawlspace/attic. Not off the pistol but in your gun safe.
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And, just between me, you, and the rest of the internet, I would recommend taking it off, and storing it somewhere inaccessible. I would argue that the “safe storage” laws from the states that have them might be a good place to start for ideas on how far, and how inaccessible the brace must be.
Or … and stay with me here, guys … it’s a perfect excuse, ah, reason to buy more gun parts. Specifically, uppers with >= 16″ barrels. Now, it’s a rifle with a short, awkward stock, but it is a rifle. For those of us that own ARs, I believe it also addresses the potential for falling afoul of being “shoulderable” even if not intended to do so (e.g. a bare buffer tube).
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Even the ATF, at least for now, says the resulting firearm is not an NFA item:
https://www.atf.gov/firearms/qa/can-i-lawfully-make-pistol-rifle-without-registering-firearm