The ATF did something kinda sorta similar with the Akins Accelerator (over $1000 stock adapter but used a spring to functionally in its reset) … “okayed” in 2002 by the ATF then in 2006 it was reviewed and deemed a “machine gun” like the bump stock. The case was argued and appealed until 2009 when the final court ruling said the Akins owners were SOL … turn in the devices (or just the spring assembly) … and you do not get 1¢ for the “amended” view of the device. If things go south on these case bumpstock owners won’t get their couple hundred bucks nor will the forced reset trigger owners.Sure ... they can try that angle. However, the property was legal at the time it was obtained .. and, in this hypotethical situation, only retroactively deemed illegal [thereby creating a regulatory taking] due to a (new/re-)interpretation of a term by a governmental agency. In such case, the regulatory taking of property that was completely legal ... and lawfully obtained while completely legal ... requires 'just compensation' under the 5th Amendment. Since they will have taken a 'machine gun' (per their own revised definition), the compensation one should reasonably seek is the amount required to replace what's been taken ... which means the value of a machine gun, all paperwork, and all taxes (plus attorney's fees) ... since that's what it would take to replace what was originally legally obtained ... and then seized via the regulatory taking. (Add in damages for the violation of one's civil rights, too, of course.)
That's a legal fight worth having ... to set a precedent for which we have a dire need. After all, a precedent that says the government (state or federal) must pay for 2A-related things they retroactively ban ... will make entities think twice about enacting such bans, as there are likely more guns than there are coffers with which to pay for them.
The bump stock ban situation is worth watching, as it could set precedent here, as property loss associated with the ATF's reversal on that front is estimated to be over $100M in privately owned assets. (Reference for the dollar figure in this article: https://www.globenewswire.com/news-...-Bump-Stock-Ban-to-Resolve-Circuit-Split.html)
As to the brace vs SBR thing if the ATF pushed the 4999 form brace owners won’t be out “hardware” … they just cant have a brace setup that gives features that the ATF wants to claim are SBR like. Owners won’t have to turn in uppers or folding adapters or such … they just would not be able to legally have ‘em all together without a $200 tax stamp.