The ATF says a rifle is “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 the explosive in a fixed metallic cartridge to fire only a single projectile through a rifled bore for every single pull of the trigger.” If the firearm started as a rifle, it becomes a “weapon made from a rifle” making it an SBR or AOW (any other weapon) if you have a barrel less than 16” or an overall length of less than 26”. You can read the official ATF stance and explanation, but the basics are pistol to rifle is fine, but rifle to pistol needs a tax stamp to keep you out of jail.