I'm sure most if not all of you have seen stories about the MAX-41A pulse rifle. Obviously, it's not for sale, since it's got a full auto M11/9 in it, not to mention the short barreled shotgun underneath.
http://www.max-11.com/TopSecret.html
BUT...
Would you be able to make a drop-in stock shaped like the M41? I don't think you could do the shotgun without a tax stamp, so let's forget that for now. Like for a Ruger 10/22 or an M1 Carbine?
Modern Thompson carbines are too long, but I suppose you can get an SBR tax stamp for a Thompson pistol and build it off of that.