As you know, a short barreled rifle (i.e., one with a barrel under 16-inches in length) is a regulated item under the National Firearms Act requiring one to obtain a tax stamp from the ATF to own. Back in December 2020, the ATF released a proposed rule then withdrew it. But we knew that it would be back.
And today, The Truth About Guns reported that the "DOJ Announces Proposed Rule to Regulate Braced Pistols as SBRs Under the NFA".
The proposed rule would "[a]mend the definition of 'rifle' in 27 CFR 478.11 and 479.11, respectively, by adding a sentence at the end of each definition to 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." You can download a PDF of the proposed rule here.
The rule does not ban pistol braces, per se, but it does address concerns that too many of the braces are being bought and used for the purpose of evading the NFA regulations on SBRs. Accordingly, the rules make use of a worksheet with various criteria to determine whether a firearm outfitted with a particular type of brace is a pistol or a firearm.
Section 1 of the worksheet is a simple size and weight sheet to determine if your pistol is large enough to reasonably require a brace.
If so, you move on to the second section which evaluates features of just the brace to determine if the brace is, per the regulation, is functionally useful as a brace or is really a stock masquerading as a brace. These include features such as whether the brace is modeled on or derived from rifle stock designs; whether it is adjustable for length of pull; whether it is molded to fit fully around the forearm or if a strap is used to secure the device to the forearm; whether it includes a sling mount; etc. Each feature (or lack thereof) is assigned a point value. Four points or more indicate that the brace is functionally a stock.
If the brace in and of itself is not a "stock", then you move to section 3 to determine if there are features of the weapon that would indicate that the weapon is intended to be shot from the shoulder, e.g.: the presence of hand stops or forward grip; using iron sights or optical sights intended for a rifle; how the brace is attached to the weapon (i.e., pistol buffer tube or rifle/carbine buffer tube); length of pull; etc. Each of these is also given a point value.
Under the example for the SB-Mini, which had it attached to an AR pistol with flip up sights and a hand stop on the handguard, the SB-Mini did not qualify as a stock under worksheet 2 nor as configured under worksheet 3.
Example SBA3 configuration used in the proposed rule.
The SBA3, on the other hand, was determined to be a stock just based on the features of the SBA3 without reference to how the rifle was configured. However, they still went through worksheet 3 just to show how that would work. The pistol with the SBA3 attached still qualified as an SBR under that section of the worksheet. Short take: the SBA3 converts your pistol to an SBR per the proposed rule.
The KAK Shockwave Blade, which uses a proprietary adjustable buffer tube, also qualified as a stock turning the weapon into an SBR without reference to how the rifle was configured. However, if it was not adjustable, that would drop the score to 3, below the threshold to be considered a stock (although you would still need to evaluate the configuration of the pistol to determine if it was, in fact, an SBR).
Obviously, this is just a proposed rule and so it could be withdrawn or modified before final adoption. And, as always, I am not your lawyer and this is not legal advice.