I have an idea for a device which would require removal of the barrel before a thread on silencer could be removed. If said barrel was under 16" but longer than with the silencer, would said weapon still be an NFA item? To clarify, while installed, the silencer could not be removed from the sub 16" barrel. To remove the silencer, the barrel would have to first be removed from the rifle, thus allowing removal of the silencer retainer device, and then the silencer unthreaded to remove. Obviously the short barrel would not be installed without the silencer.