Depending on what state you reside in, there are ways to make the gun legal. The gun can be registered as a short barrel rifle and you can do what you want with it. Put on the evil front pistol grip and folding rear stock. Even make it fully automatic again, providing that you can find a legally registered sear. All this gets really techno though, in order to put on these parts, the gun had to have been built before 1994. And a gun like I am describing can cost between eight and ten thousand dollars. The civilian model, the SP89 (it is technically a pistol, no rear stock and front grip. Semi automatic) cost around three thousand.
Robert