Looking around on the internet four or five years ago, I found a site that sold miniature cannons for desktop display. I thought they looked pretty neat, but wasn't about to part with my allowance to buy one. After a quick stop at the local hardware store later that day, I was the proud owner of some 3/8" galvanized pipe and a threaded cap. I screwed the cap onto the end as tight as I could, and drilled a 1/16" hole through the cap and into the pipe near the back. I then took the cannon outside and put it in a small vice and dropped a firecracker fuse down the touch hole. I put a few grains of black powder in the barrel and a ball of newspaper over that. I lit the fuse and jumped back into the shop. I figured the newspaper wouldn't offer much resistance and the cannon wouldn't blow up. I was right, as the cannon gave a pretty dismal "poof" and the ball of newspaper landed 3-4 feet in front of the barrel. Not very impressive, but enough to build my confidence.
Through the afternoon, I slowly increased the amount of black powder until I was using 15 grains and a tight fitting wad of newspaper. The cannon was holding together nicely and giving off a loud boom with lots of smoke and fire. This was exactly what I wanted, so I decided to make a wooden carriage for it.
I didn't use any plans or take many measurements, I just built the carriage around the barrel in the style of a naval cannon. The carriage is made from scrap pine, with some spare wooden wheels from another woodworking project. The metal band that holds the barrel down was cut from an eye bolt, and I had to bend it to fit a little better. There's also a wooden wedge that holds the barrel secure. It needs replacing every now and then, and I'm on my third or fourth. Perhaps if I made it out of hardwood it would stand up better.
Black powder is pretty corrosive, so every time I shoot it I take the barrel off and wash it in hot, soapy water. Then I dry it and spray a light coat of Rem Oil on before putting it away. It still manages to rust though, but so far it hasn't been much of a problem. The pipe was labeled as 3/8" but after measuring it recently, the inside diameter appears to be 15/32". I'm not sure if that's because of wear and corrosion, or if 3/8" pipe is actually made a little bigger. It will be interesting to see if it changes, because this is the first time I've measured the caliber.
Needless to say, this cannon is pretty popular with guests and my dad always has me fire it off for them. I'd imagine it's been shot close to 100 times, and still going strong.
This is just what has worked for me. I don't have any plans, but it should be fairly easy to work out how I made it through the pictures. If you decide to make this, your safety is up to you.