Regarding an inquiry about a short-barreled shotgun design that incorporated the SB15 brace (which is intended to allow a shooter to fire an AR-style pistol with one hand using a Velcro strap to attach it to the arm, and has heretofore been classified as a pistol and not a short-barreled rifle), Acting Chief of the BATF’s Firearms Technology Branch Max Kingery, responded in a letter stating:
“The submitted weapon, as described and depicted above … is not a ‘firearm’ as defined by the NFA (i.e., not an NFA-restricted weapon) provided the SigTac SB15 pistol stabilizing brace is used as originally designed and not used as a shoulder stock. “However, should an individual utilize the SigTac SB15 pistol stabilizing brace on the submitted sample as a shoulder stock to fire the weapon from the shoulder, this firearm would then be classified as a ‘short-barreled shotgun.'”
So he’s saying the legality of possession of the combination literally depends on how you individually hold it.