This also happens to be my first instructable. I hope you enjoy!
Here's how to make one:
Step 1: The Parts
6 feet of 3" diameter PVC pipe
1 1 1/4" PVC ball valve.
1 3" PVC end cap
2 3-2" PVC reducers
2 2 - 1 1/4 PVC threaded reducers (the 1 1/4" end needs to be threaded to fit the ball valve)
1 Husky 12V inflater with light
All the above can be bought at Home Depot for about $40.
You'll also need an old bike tire.
Tools for this project:
boxcutter or X-Acto knife
Step 2: Cut the PVC
Step 3: Modify the End Cap
Next, drill a hole about the size of the stem of the valve into the center of the end cap. A 5/16" bit worked well for me, but check for yourself before you drill. The important thing is that there's a really snug fit when the stem goes through the end cap. Remember, there's going to be 100+ psi of pressure there.
Once you have a nice snug hole for the valve to go into, put some super glue on the rubber surrounding the valve, and stick the valve into the end cap. Smooth out the rubber so there is a tight and smooth seal against the PVC.
Step 4: Prime It and Glue It
Once the primer has dried, glue one 2-1 1/4" into one 3-2" reducer. Then do the same to the other reducers. Glue one reducer assembly to the 4 foot barrel, and the other to the 1.5 foot air tank. Finally, glue the end cap to the other end of the air tank.
Watch out for the PVC glue. It stinks to high hell and can really mess you up. And don't smoke while gluing either.
Step 5: Put It Together
Step 6: Load It Up and Launch It!
Once you've got it fully charged, load the cannon up with whatever you please. I think water balloons are a lot of fun; just make sure you put a soda can or something in the bottom so the air pressure doesn't pop the balloons. Aim and fire! You can literally launch anything you can fit in the barrel.
You can slap a really cool paint job on it, too. We used a plastic primer and some simple spray paints to do a camo design. Then I did some air brush work to christen it Thompson's Nightmare after our favorite attorney from Florida.
Just keep in mind one thing: anything you launch out of the cannon is coming out at 90+ mph. Don't be stupid. Have fun!