This little cannon is powered by bursting a rubber diaphragm with a bicycle pump. I cut the diaphragms from an old bicycle inner tube. You can double them up or burst one at a time. It uses 40mm foam practice golf balls for ammo. You can also use it with just the pump but it doesn't fire with near as much force.
UPDATE: Apparently you can use more than just a rubber diaphragm. I've also tried duct tape and aluminum foil. It works great.
Step 1: Get the Parts
Plumbing parts come in all shapes and sizes. These are no different. You could make the same set up bigger or smaller. You could even modify the barrel for a smaller diameter and use it for paper rockets. What you see here cost me $14.00.
You'll also need a base and a way to attach it. I'm using the base form an Estes launching pad. The support bracket came from an old portable fan.
Step 2: Install the Valve Stem
I started by drilling a pilot hole with the end cap mounted in my lathe. From there I moved it to the drill press and drilled it out with a 1/2" bit. I used sand paper to ease the edges of the hole.
Having a valve stem tool is helpful but not critical. Before pulling it in place I added a little grease to ensure a proper seal.
Step 3: Screw the Parts on
Since only one side is pressurized you only have to worry about a seal at the valve stem and at the threads to the end cap. Before threading the cap into the union coupler I applied silicone grease. For the exit side I simply threaded the adapter on hand tight.
Step 4: Prep the Barrel
I cut a section of pipe about 3" long. I then used a razor blade to round over both the inside and outside edges. This allows the foam balls to load without getting caught up. To smooth out the finish I used sandpaper. All that's left is to push the barrel into the adapter.
Step 5: Cut the Diaphragms
This part is pretty straight forward. Use a pair of scissors to cut a section of bicycle tube out. Unscrew the coupler and use the male half as a template to cut a disc out.
Step 6: Testing... Testing
Without a foam ball in place this cannon is "call the police" loud. Check out the video and you'll see the air burst push the camera back (see the intro video at 1:05). Out of all the discs, only one ruptured with a slit. All the others took a star shape. Sometimes I'd see pieces of inner tube in the barrel. So keep in mind more than just a foam ball could come flying out. Don't shoot your eye out.
Step 7: Mount the Cannon
I used the bracket of an old portable fan for a mount. I hot glued it on and added a couple zip ties for support. The base is an Estes model rocket launch pad. After a little machining on an aluminum tent stake it fit right in the base (see the last picture).
Finally I painted it with rubberized undercoating.
Step 8: Launching Ammo
The barrel has an inner diameter of 35mm while the foam golf balls are 40mm wide. This makes a snug fit which is great for launching. While I'm using an old bicycle inner tube for diaphragms you could also use these.
It works without the diaphragm but it doesn't go as far. As tempting as it might be, don't shoot other people with this.
Thanks for reading.