OTMG, I am not sure where you get the idea that attaching a shoulder stock to a pistol (like the Luger) is manufacturing an assault rifle. (It is, of course, a short barrel rifle.) An assault rifle is defined as having (among other things) a folding or collapsible stock; a detachable pistol stock is not mentioned because it is covered elsewhere. I think that combination could only be an assault rifle if the pistol also had two other "bad" features.
FYI, I am reading from Title 18, USC, so if I am wrong, would appreciate your citing the section.
In other words, I don't think there is a difference between a shoulder stocked revolver or single shot, and a shoulder stocked (ordinary) pistol.