Mark, if the ruling is not overturned, you cannot legally attach anything that can be shouldered unless you first register it with the ATF and pay a $200 tax stamp. I would wait until the dust settles before putting money down on something you won't be able to use as intended.
Stay safe & have a great day!

Hi Mark and welcome back. Until the proposed Brace ban is resolved, adding a Brace or Stock without NFA registration and $200 Tax Stamp is in the air. If you do end up having to register, just go with the full Shoulder Stock and if not, the Brace. Cheers and stay safe!

If you want to attach a brace or stock to it, you have to go through the process of registering it as an NFA weapon (SBR). You can't legally attach anything to it without the proper approval or you'd be committing a major crime.