My comment as submitted. I think I see their "intent," and as usual, they have a ham-handed, complicated response:
docket number (ATF 2020R-10)
I do not own nor have any interest in arm braces myself. I understand the published criteria and subjective nature of the difference between a brace that is functional and one that may be a workaround for an actual stock.
However, ATF should consider that constant flexible and re-interpreted definitions serve at one level to create contempt and distrust of the agency. One outcome of this could be greater deliberate disregard of the law and refusal to attempt to comply. Another significant risk, one that has been observed, is the inadvertent violation of an increasingly complicated set of criteria beyond the grasp of the casual, non-expert firearm user. In the former case, deliberate violations of fine points of law may lead to greater violations of the NFA and other firearm law. In the latter case, people with no criminal intent may be found guilty of serious federal crimes over purely administrative details.
ATF notes that some firearms with braces are long and heavy enough to not function well with a brace. However, such weapons are also difficult to conceal and therefore the risk of deliberate violation of the INTENT of the law regarding SBR/SBS is very low. Such crimes, if they occur, are usually part of another violation and can be punished as such.
ATF's criteria seem to favor shorter weapons with smaller cartridges, that ironically function better in a concealed role. However, with a brace mounted, these become less concealable, and this supports the intent of the NFA proscription.
I would urge that any brace be considered to be exactly that--a brace.
A shorter brace on larger firearms does not function well as a stock due to eye-relief and recoil. While there would be an administrative violation in such case, the practical ability to commit a crime of violence isn't great.
A shorter brace on a bona fide pistol makes shouldering cramped and ineffective, and hinders concealability.
ATF should therefore research if a maximum possible length of pull of say, 10-12", serves the practical purpose of supporting a small weapon for one-handed firing, while limiting the ability to convert a firearm into a de-facto short carbine or shotgun.