The ATF has recently issued a General Notice titled Objective Factors for Classifying Weapons with "Stabilizing Braces". It's stated purpose is

ATF is publishing the objective factors it considers when evaluating firearms with an attached stabilizing brace to determine whether they are considered firearms under the National Firearms Act (NFA) and/or the Gun Control Act. ATF publishes this notice to inform and invite comment from the industry and public on the proposed guidance prior to issuing a final document. Upon issuance of final guidance, ATF will provide additional information to aid persons and companies in complying with Federal laws and regulations. This notice also outlines ATF’s enforcement priorities regarding persons who, prior to publication of this notice, made or acquired, in good faith, firearms equipped with a stabilized brace. Finally, this notice previews ATF’s and the Department of Justice’s plan to subsequently implement a separate process for current possessors of stabilizer-equipped firearms to choose to register such firearms in compliance with the NFA, including an expedited application process and the retroactive exemption of such firearms from the collection of NFA taxes.

The public comment period ends Januray 4th, 2021. Keep in mind that while public comment is being solicited, swearing, imporperly filled out forms, form letters or substantially similar comments may be ignored.
Submit your comments here on days other than Tuesday or Thursday
Submit your comments here on Tuesday or Thursday