I am going to call this Revision 1. If I find something important to be missing, I will bold the new text and bump the revision.
I like having interesting firearms. I go through the the annoying paperwork because I enjoy the fancy toys. I am sharing what I know, rather than just telling you to research things because time is running out on the current ATF offer. You need to know your local laws. I don't know where you live, much less what your local restrictions are. I have only done any of this in counties in Arizona and Utah. I am not an FFL and I am not a lawyer. I am an individual sharing what I have learned as I understand it.
This post is to detail some information for how to deal with "Firearms with Attached ‘Stabilizing Brace.'" The information in here is not complete, but is given as general guidelines of what to expect and hopefully enough information for you to decide what you want to do. The intent of the incomplete information is to help you decide what path you need to fully learn about and to prevent you from wasting limited time learning about options that you don't want.
There are multiple methods of compliance, all with various pros and cons, just like every other choice in life. One of the methods is to register a pistol with a stabilizing brace as a Short Barreled Rifle (SBR), using yourself as a builder and filing what the ATF calls a Form 1. The ATF currently has a "much nicer than normal" path for doing this for firearms that are currently affected by the Ruling. The path is easier, and they are waiving the $200 stamp fee. How is the path easier? You still get to keep your firearm while you are waiting for paperwork. After this special window, or currently for any firearm other than ones that meet the current, special, circumstances, you don't get to build/assemble/take-home/whatever until AFTER your paperwork is cleared. Please read the rules on this part very carefully; I am not fully stating all requirements, I am giving signpost directions.
While some of the other options are fairly self explanatory, filling out ATF paperwork for the first time is like filing your taxes for the first time. Just like your taxes, you can also pay a fee to a professional to assist with the paperwork. None of it is hard, but all of it has to be perfect. As of this post, if you want to go this route and take advantage of the "much nicer than normal" path, you have eight days remaining (5/31/2023 and this path goes away, and only the more cumbersome option remains, and you have to pay the $200). There are multiple things to line up in about a week, if you want the option, consider asking a local FFL that deals with suppressors and the like to assist. Some don't charge very much for helping and they already know some of the names you need to deal with.
Read this: https://www.atf.gov/rules-and-regulations/docs/undefined/eformone-externalguidancewithqapdf/download
Minimum things you will need.
2 fingerprint cards, or an electronic file. Paper fingerprints can be sent up to 10 days after filing your form, the electronic file (EFT) can be uploaded immediately.
Passport sized photos
You will be filing with the ATF, and sending notification paperwork to the CLEO (The chief law enforcement officer is the local chief of police, county sheriff, head of the state police, state or local district attorney or prosecutor).
There are multiple good answers to the question on the form regarding "for what purpose." I have seen (have) "For collection and pleasure," and "For all lawful purposes." The answer in that box is lawyer-speak, so choose wording carefully. Again, paying a little to have a local FFL help is handy.
You will need to engrave the firearm as the builder. The ATF has specific rules for that as well. I do not have an ATF link to that, yet, but google can hel you find info, also.
The target audience of this group will need to decide to register the SBR to a trust, or to themselves as an individual. Rules for having/using a SBR are a little cumbersome. TL;DR of the options:
Individual: that's you and that's it. You can let somebody else shoot it at the range, if you are there, watching, and ready/able to maintain control. Letting somebody borrow it without you is a federal no-no. If you move, this still goes with you, because it is registered to you, no matter where you are. When you die, there are rules for allowing your spouse to sell things off through proper channels. If you don't have a spouse, things get trickier, but you don't care because you are dead.
Trust: this lets you do a "family plan," where you can add other people to the trust. You can be a trust with only you in the beginning, and do paperwork to add people later. Your trust has an address. If you need to move, you have paperwork involved with changing the trust physical location, and you should be planning on having the trust location be where the firearms are usually stored. Anyone in the trust, with permission from the trust, can take the SBR to the range. Adding people to the trust takes fingerprinting and pictures, just like filing as an individual. There are online trust assistance services; they are not that expensive but may take a couple days. When you die, the trust can continue without you.
If you pick one route and later want to change to the other route, you need to re-register the firearm, pay a new $200, and engrave it again. For something like an AR, registering a new lower is likely a simpler process, based on the low cost of a lower.
ATF account/login https://eforms.atf.gov/login
If you go trust route, even with the trust, you have to file a responsible person form that is very much like filling out a form for an individual. Info here: https://www.atf.gov/firearms/docs/form/national-firearms-act-nfa-responsible-person-questionnaire-atf-form-532023
If you can and do go through any of this, PSA, get a suppressor. As expensive as they are, they cost about as much as six months of hearing aid life. Be nice to your knees and your ears. You will miss them both when they are gone.