Well, I spent way more time on research today than I should have. I can find nothing in either the National Firearms Act or the Gun Control Act of 1968 referencing the required dimensions for a handgun (pistol or revolver). Nothing about barrel length, overall length, height or weight. Online searches produce nothing in these documents or any other as far as Federal Law. There are only references to, paraphrasing, “intended to be fired with one hand”. There ARE dimension references for qualifying as a rifle; it must be over 26” overall with a barrel 16” or longer. In a proposed worksheet it appears the ATF has awarded themselves a poetic license to assume then that any AR type handgun must be both shorter than 26” overall with a barrel shorter than 16” to even be considered for legal use of a stabilizing brace. I find no law that mentions, grants, or approves that assumption. If someone cares to enlighten me a link will be appreciated. This is a pistol. The stabilizer is an admitted “accessory” by ATF.
Other items I can across are equally dismal. Many references in ATF documents to “sporting use” and “determined by the Director”. Neither statement passes Constitutional muster. Nothing by our founding fathers about “sporting use”. Nothing about law being determined by any director. Nothing about a created department determining what is and isn’t covered by a very simple 2nd Amendment. Much more to read that I have not referenced.
The first link goes thru the Factoring Criteria ATF used to set up a worksheet for determining what is a legal AR pistol WITH brace. The second link goes to a much shorter version. The third link is the proposed worksheet. Public comment on the worksheet was closed in September, 2021. Who knew. So here you are; the high points of what I found:
Factoring CriterisShort VersionATF Worksheet