There's No Guidance on What Couldn't Be Illegal
Given the public interest surrounding these issues, ATF is proposing to amend the definition of “rifle” in 27 CFR 478.11 and 479.11, respectively, by adding a sentence at the end of each definition. The new sentence would clarify that the term “rifle” includes any weapon with a rifled barrel and equipped with an attached “stabilizing brace” that has objective design features and characteristics that indicate that the firearm is designed to be fired from the shoulder, as indicated on ATF Worksheet 4999.
While this might seem (and be) quibbling over definitions, a "rifle" measuring with less than an 16 inch barrel or 26 inch total length is regulated under the NFA1934. This is kind of bullshit, but it's the sort of bullshit that can land you years in federal prison if you end up An Example, and even constructive possession (that is, having the parts to make a regulated gun, even if the individual components or working combinations of those components would be legal separately) counts. The proposed rule has no grandfathering, nor a waiver of the 200 USD per unit tax. It's not known exactly how many stabilizing braces in this category are out in the wild, but numbers between 3 and 10 million have come out of government offices and don't seem unreasonable, and there's pretty likely more than a million individual owners.
For some earlier discussion of an earlier iteration of the rule, see some previous discussion on an earlier implementation here).
Of course, in addition to being filled with a variety of arbitrary or vague terms (a user adding a RedDot to a gun with 11" length of pull would magically turn a legal handgun into an SBR), on the top of the proposed ATF Worksheet 4999 :
The Bureau of Alcohol, Tobacco, Firearns and Explosives reserves the right to preclude classification as a pistol with a "stabilizing braces" for any firearm that achieves an apparent qualifying score but is an attempt to make a "short-barreled rifle" and circumvent the GCA or NFA.
That is, there may be a lot of guidance on what would be illegal. There's no guidance on what couldn't be.
This is a bit of a theme with recent firearms-related regulations.
The ATF's proposed "ghost gun" and split receiver rule doesn't specify exactly where the line between blank material and federally-serialized firearm should be, just that it's somewhere after "eight-hour working day in a properly equipped machine shop was readily restored to shoot" and before "when restoration required master gunsmith in a gun shop and $65,000 worth of equipment and tools". Apparently, the less said about their 'split receiver' definition the better. The Biden administration's proposed Director of the ATF, in addition to his many other problems, supports a ban on "assault weapons", which he also can't actually be bothered to define when asked by Senators beyond mumbling about .22s maybe not counting. The DoJ model "red flag" legislation doesn't mandate a specific standard of evidence, but it quite happily suggests stripping a constitutional right based a mere preponderance of evidence standard for at least a year (and that's the 'due-process-attentive' one; the other option the DoJ recommends states make available is a fully ex parte hearing). The Trump-era "bump stock" ban was a vague mess that by its strict read didn't even consistently cover its target, and from a read broad enough to do what the ATF wanted could as consistently ban a rubber band or a shoestring.
Some of this is, bluntly, stratagem. Actually providing clear standards of lawful behavior that people can follow, or strict due process, or genuine statement of a policy proposal isn't a bonus, anymore; it's a vulnerability. Genuine policy proposals face criticism over boring matters like constitutionality or practical problems over the number of effected people. Actual due process can let the guilty slip through with the innocent. Clear standards run into the risk that someone might follow them, but not do what you want.
Which seems a little odd, when there's not that much intent or capability to actually enforce these rules. The BATFE isn't going to be able to meaningfully get everyone (or even than many) owning these covered guns, any more that it could require a NICS check for everyone buying aluminium stock. Nor are there any odds that the enforcement that does exist will reduce actual violent crime, given the minimal relationship between the newly regulated topics and actual violent crime or to the BATFE's practical enforcement role.
That's not the point of the rules, any more than getting existing guns "off the streets" would be. Given how quickly the ATF will drop suits when its regulations are challenged, it's hard to see a deep interest in protecting the public. It will enjoy the burst of additional funding from the habitually law-abiding, and then bring perhaps thirty-odd cases a year To Make Examples, and threaten perhaps a hundred times that to make useful informants. The deeper point...
There's an exchange I've seen a few times, gun advocates point out nil compliance and 'boating accidents' and burying rifles in their backyard, and gun control advocates say they'd count it as a win! They might prefer every single gun be melted down into ugly statues, but wrapped in tarp and three feet underground is not, from their perspective, that bad a second place.
I think there's a social equivalent intended here, especially given the prominent push to use 'soft power' on gun-related matters, either through civil tort or externalencouragement. From that perspective, even regulations which are never applied under the police power are still exceptionally useful, and the vague or arbitrary nature is, if anything, only enhancing that. How many bank compliance departments wants to play games with possible short-barrelled rifles? How long will a credit card processor? A social media company to try to parse things that could perhaps be unlawful manufacturing (or could be legal, but 'encouraging' or 'useful' to illegal behavior)? How about after a pressure campaign?