Easy answer..... The ATF said the arm brace is not designed and intended to be a shoulder stock. It is meant to brace the arm for another point of contact and stability firing as a pistol. There was a time when they said shouldering it redesigned it and therefore requires papers, but now they released another letter saying the misuse of a device does not constitute redesign. So basically, they don't like shouldering a pistol brace but aren't currently doing anything about it.

I have a pistol brace 300blk that is my camping gun. Biggest threat by me is over zealous rednecks and coyotes. I use the brace because I don't have to send papers for interstate transportation of a sbr.

Bottom line.... get one, have fun, and don't be a tool publishing videos of you doing your thing.
The below link is a simple google search of "arm brace letter". It has the original opinion and updated letters giving their current opinion. Their opinion is always subject to change, so make up your own mind on arm braces and be smart.

https://www.google.com/search?q=arm+...hrome&ie=UTF-8