My idea for a prison gun is basically a breechloading coach gun like pic related. For the shotgun shells, you can make paper mache with newspaper for the shells, with a hole sticking out the rear for you ro insert the primer, a cardboard circle cut out to put inside the shell at the back with a hole for you to stick the primer through, a felt wad in front of the propellant and finally the shot (which you could use about anything, but for this sake we'll just say to use the lead from a AA battery). For the primer, it simply consists of a stick of graphite from a pencil and the propellant charge is thermite made from aluminum shavings and rust from a prison workshop. If thermite doesn'r have any contact with air, it may become explosive. You want the paper mache/plaster of paris to be thick and hard as clay for the outer shell. As for the gun itself, for the hammers it uses two alligator clips connected to wire and a 9 volt battery which is removable from inside the stock of the gun, so that when you pull the trigger, the alligator clip will come down and bite the graphite, causing it to burn or explode, setting off the thermite propellant. Or at least I think that'll work. I know graphite will burn at high temperatures. Anybody with better knowledge can perhaps give me a better idea.
As for the shotgun itself, you can make the two barrels out of an iron bedframe, and as for the inner workings and trigger mechanism, I don't really know that. Just make the frame, trigger and barrels out of iron from bed posts.
Any ideas?