You do not register any gun except NFA items. An AR pistol is not a NFA item.
Under the letter of the law, if you shoulder it, you have created a NFA item. How you would get caught, I have no idea. Unless you did something like post a photo or video on the internet.
Just for clarification, not really "under the law", but more of a recent ATF opinion letter that said they would consider any pistol that had a brace placed to the shoulder as using the brace for something outside the approval parameters, and therefore could be viewed as changing the approved pistol with brace to an unregistered SBR.
Under the letter of the law, it is a pistol, plain and simple. Placing it to the shoulder does not magically transform it into a rifle like some might like to think.
Anyway, they "loophole" is when you buy an AR pistol that includes the brace at the time it is purchased. Therefore you do not modify it by attaching the brace after the fact, and it is not intended to be shouldered, according to the manufacturer you bought it from. Therefore, if you misuse the product and have not altered the product, then you have not created a new product through it's misuse.
Simple, right? More like ridiculous.
If I were to take my Buckmark 22 and put it against my shoulder and pull the trigger, it would still be a Buckmark 22 and a pistol.
Common sense will prevail, eventually.
Here's the original ATF letter stating that shouldering a AR pistol, even with a brace, does not mean you have manufactured a NFA item.
http://truthaboutguns-zippykid.netdna-ss...ATFE-LETTER.jpgHere's the ATF letter stating that misusing an item might lead you to "create" an entirely new item/class and therefore commit a felony. Now they're saying any previous letters stating otherwise are rescinded. Silly, but true.
https://www.atf.gov/file/11816/downloadMore on the subject -
https://www.gunsamerica.com/blog/can-legally-shoulder-sig-arm-brace/http://www.thefirearmblog.com/blog/2016/...might-be-wrong/