Our definitions are a little different than the federal versions. For instance this is the federal definition of rifle.
Federal definition of Rifle:
The term “rifle†means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire only a single projectile through a rifled bore for each single pull of the trigger.
vs TN definition:
"Rifle" means any firearm designed, made or adapted to be fired from the shoulder and to use the energy of the explosive in a fixed metallic cartridge to fire a projectile through a rifled bore by a single function of the trigger;
The federal version doesn't have the adapted language in it. This is the real meat of what is bothering me about the arm brace. I think if a pistol with an arm brace can be considered a rifle then it can be considered an SBR. I also think this is an all or nothing type of deal. The law regulates possession not how it is used so the brace is either legal or not.