Sometimes, my wife tells me it's amazing I have all of my fingers and eyes. This is the sort of thing she is talking about. This is a line drawing from a U.S. Military manual called "Improvised Munitions" and it shows how to craft a 12 gauge shotgun out of scrap lumber and steel water pipe. Very nifty, actually. Just to be clear, I do not advocate building something like this, as not only is it illegal where I live, but would be substantially dangerous to your person were you to actually use such a thing.
On the other hand, if you built it set up to be a blackpowder shotgun, the legality issue changes, at least where I live. Still dangerous though. Don't try this at home.