I made this out of pure nostalgia. Most of my friends and I made one of these when we were about 12 years old. They were surprisingly effective. To fire, you loaded a firecracker and laid the fuse in the slot filed into the barrel (3/8 ID galvanized water pipe), then screwed the cap on. A pencil served as a ramrod and a wad of toilet paper went on top of the firecracker followed by about 10 BBs or a 00 buckshot and another wad of TP. Then light the fuse, aim, and a few seconds later "BANG!!!" We made our stocks from left over house construction tongue and groove lumber and the barrel was held in place with a generous wrap of cloth electrical tape.
I STRONGLY recommend that you not attempt to fire this thing if you decide to build one. Mine doesn't even have the fuse slot filed into the barrel.