AR pistols have been around for at least four decades, but about a decade ago they took off in popularity due to the introduction of the pistol brace. The brace was designed to allow disabled veterans the ability to fire an AR pistol with the pistol strapped to their arm. The ATF approved the design, saying it was fine and not subject to the National Firearms Act (NFA).
Within a few years, the agency changed their mind and put out a memo informing people that if they shouldered them, the braced pistol would now be considered an SBR (short