Denis-
The only things that get permanently placed on the registry (at least until they are destroyed) are machineguns and suppressors. Even a destructive device can be put on the registry and later removed from the registry (or vice versa). For example you can buy an M203 or Lahti receiver as a title I firearm, but if you add the 40mm barrel (or 20mm in the case of the Lahti) it must be registered as a DD. However, if you convert the Lahti to .50 rather than 20mm, it can stay as a title I firearm, or be converted back from a DD to a title I. Things can get a little hinky taking it off the registry when its a SBR originally manufactured as a pistol, but it can be done.