You raise some valid questions. I don't have all the answers. The ATF has a section for FAQ on rule 2021R-08F. You can also peruse the entire rule on that site. I suggest you look there.So then they need to turn in their uppers and other gun parts?...I mean, if they own any stocks, braces, what have you, they all "could" be a part of a build? What if they own other AR's uppers or rifles that are 16"? Are they not allowed to own a stock that fits them? The ambiguity of the whole thing is why I already said folks should just put a 16" barrel if they don't plan to apply for stamp...but a stock or brace is still just a piece of plastic that can be used on any rifle. If I had one, that's what I'd be doing and just make it legal...easy peasy. I suppose if you had a short upper you could either keep it as a legal pistol (assuming they clarified pistol buffer tube is fine), or you could store it (the upper as a part) at someone else's house. But as I understand it, a regular stock or brace will NOT fit a pistol buffer tube? So if the AR had a pistol buffer installed then it would not be able to receive a stock or brace...don't you agree?