The legal battle for the pistol braces have been pretty wild in the last two years. As of January 13, 2023, the ATF has released its official stance on the pistol braces Rule 2021R-08F , and this guide is your reference for list of ATF approved pistol braces to go on your pistol configured "long" guns.
Please read the rule 2021R-08F on ATF's official website. It basically says if the rifle’s barrel is under 16 inches and it wears a brace, it’s an SBR under the new guidelines.
Check out best AR15 pistol braces here
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However, in late 2023, the ATF brace rule is dead. This means you can all go back to putting your favorite SB Tactical braces on your AR pistol or other pistol variant guns.
Please check out the Britto VS ATF case for more details.