In this Instructable I will be describing another design for the all-popular pneumatic (air) cannon. My version is similar to many of the other ones on this site, but my design was my idea so don't say I copied it from you. This is very simple to build, costs little, does not require alot of skill, and is fairly "safe" (especially if you are careful with it).
This is a weapon and can be very hazardous if used irresponsibly or used at all. NEVER, EVER point this at anybody because, despite its benign appearance, It could really mess you up. What you do with this is your responsibility, not mine. It is advisable to check your local laws before building this, it may not be legal where you live. Always treat guns liked they are loaded. Use eye protection. Use common sense. Plastic bottles like the one used in this Instructable can be prone to violent failure which could result in injury. Please take adequate safety precautions and use a PVC air chamber when you intend to use higher air pressures than what has been recommended. Happy shooting!
Step 1: Materials
1/2 inch pvc piping (the higher rated, the better)
1x 1/2 inch T connector
1x 1/2 inch ball valve
1x empty 2 liter bottle
Duct or electrical tape
Bike tire valve
Step 2: Cutting the pipe
Now you need to cut your PVC piping to the right lengths. The lengths are approximate.
1x 2 inch
1x 5 inch
1x barrel: this can be any size you want: longer = more accurate, shorter = easier to carry
You can cut the pipe with a hack saw unless you have special pipe cutting shears.
Step 3: Building the Chamber
Take the 4 inch section and use a few small pieces of masking tape and wrap it around the pipe at one end. It should only go around the pipe about 2/3 of the way and come up the pipe about an inch. This will make a tighter seal on the bottle. Now slide the pipe into the bottle to the top of the masking tape. If it is not a tight fit, use more masking tape. Then wrap duct tape tightly around the bottle neck and pipe to seal it off completely. Blow into the bottle and listen for leaks. Seal them. For extra safety, cover the entire bottle in one or two layers of tape.
Step 4: Assemble the pump valve
Take the valve and push it into the 1 inch piece. The lip of the valve should firmly hold it in the pipe. the pipe should be short enough so that the connection for a bike pump sticks out of the end when the valve is firmly pressed in. If it isn't, cut the pipe shorter. Take the pipe with the valve in it and push it into the T connector as shown.
Step 5: Put it together
Now for the final construction step: assemble all parts! Line up the parts as shown in the picture and firmly push them together. If you want, you can use special PVC glue to hold them permanently and to give a more reliable seal. If you don't, it is easy to disassemble the cannon for cleaning and new configurations plus it will be easier to carry. A pro for leaving it non-glued: If you pump it up too much, it won't explode, but instead, the air chamber will just pop off, releasing the deadly pressure.
Step 6: Ammo
Ammo possibilities are restricted to whatever fits in the barrel.
Small-ish grapes / tomatoes
AA batteries (very stupid-- do not use)
Step 7: Now to shooting
To use the cannon, take a bike pump and attach it to the bike tire valve. Pump it until the bottle is firm like a basketball. Slide whatever you are using as ammo down the barrel. Then hold the bottle against your hip and aim the barrel somewhere safe. When you are ready, twist the ball valve open as quickly as possible and watch your projectile fly into the sky!