It is still against BATF regs to have a shoulder stock on any firearm with less than a 16" barrel. The only way around this is to register your frame with the BATF as a NFA "short barrel Rifle". I would be curious if, once so registered, you could make it up into any combination you wish. A $200 registration wouldn't be all that outrageous if it let you change barrels and configure it any way you wish, i.e., go from rifle to pistol, to short-rifle, and in any caliber you wish. The only fly in the ointment would be, it would cost $200 to transfer that frame to anyone else, forever after. Oh, well,,,,,,,,,,,,,,,,,Bug.