The two videos below will have you dreaming of pipes. They demonstrate a two piece shotgun with one pipe containing a firing pin fitting over the pipe that serves as the barrel.
It may be crude, but I find the simplicity of the design appealing. Plus, parts can be had from your local hardware store, home improvement big box store or perhaps Online Metals.
A Wikipedia article on improvised firearms notes that such a shotgun called the paliuntod was used by insurgents in the Philippines during World War II during Japanese occupation.
Take a few minutes to watch these videos.
Explore improvised firearms further in Homemade guns and Homemade Ammo and The Practical Guide to Man-Powered Weapons and Ammunition.