See my link to Q's site below, it has the ATF letter. There are a bunch of 2A guys making videos about it. Short version, what was fine as of 2017 as ruled by ATF
https://www.thetruthaboutguns.com/atf-its-legal-to-shoulder-an-ar-15-pistol-equipped-with-an-sb-tactical-arm-brace/
Is suddenly SBR by regulatory fiat. Of particular note is that 2017 was regarding SB Tactical braces, and in the Cease and Desist letter to Q, ATF notes that their proprietary stock is manufactured by SB Tactical.
See my link to Q's site below, it has the ATF letter. There are a bunch of 2A guys making videos about it. Short version, what was fine as of 2017 as ruled by ATF https://www.thetruthaboutguns.com/atf-its-legal-to-shoulder-an-ar-15-pistol-equipped-with-an-sb-tactical-arm-brace/ Is suddenly SBR by regulatory fiat. Of particular note is that 2017 was regarding SB Tactical braces, and in the Cease and Desist letter to Q, ATF notes that their proprietary stock is manufactured by SB Tactical.