First, welcome to the forum. Stop over by the intro thread and say hello.
Now as far as legalities- not a lawyer. We had a couple, but some of the questions here left them curled up in a fetal position and rather catatonic. Their recovery is possible, but only if we keep the questions short for a while.
First, Federal law does not prohibit you from building a gun for yourself (can't build them to sell without licenses). The gun you build cannot be a gun covered under the 1934 NFA unless you get permits first (Short barreled shotgun, etc). There MAY be state laws you need to follow as well.
Now,having said all that, a pistol that can fire .410 ammo through a RIFLED barrel is not a shotgun, and is no contrary to Federal law. It is .50 cal or less (larger triggers Destructive Device category) and it is rifled.
You CANNOT use the frame of a rifle or shotgun to start your build- that would be "Making a short barreled rifle" . BTW, already been done- look up Leinad or Cobray. http://www.thegunnetwork.com/classifieds/386/Leinad_Cobray_OU_Double_/