From The Truth About Guns:  http://www.thetruthaboutguns.com/2017/04/foghorn/atf-its-legal-to-shoulder-an-ar-15-pistol-equipped-with-an-sb-tactical-arm-brace/

Nick Leghorn
April 24, 2017

In 2013, SB Tactical introduced the AR-pistol “stabilizing brace.” Navy veteran Alex Bosco designed the brace for a wounded warrior who had lost an arm in combat. The ATF approved the device for general use. The People of the Gun soon realized that an AR-15 pistol equipped with a stabilizing brace could be used as a short-barreled rifle (SBR). Justifiably worried about falling afoul of onerous, expensive, time-consuming and extremely punitive NFA requirements, more than a few law-abiding Americans wrote the ATF asking for clarification.

In January 2015 the ATF released an open letter answering their concerns. The Bureau stated that the act of placing an AR-15 pistol on the shooter’s shoulder constituted a “redesign” of the firearm, instantly turning it into an unregistered SBR. You could fire a brace-equipped AR pistol from your hip, your chest, even your forehead. Shoulder it, though, and you’re committing a federal felony.

Sales of pistol arm braces dropped as concerns about legality grew. Despite the fact that the ATF would have had a very difficult time defending their decision in court, no one wanted to be “that guy” to test it.

Complete article at:  http://www.thetruthaboutguns.com/2017/04/foghorn/atf-its-legal-to-shoulder-an-ar-15-pistol-equipped-with-an-sb-tactical-arm-brace/