This project is also documented on my website at: http://www.mbeckler.org/pneumatic_cannon/
Step 1: Safety Warning!
If you only remember one thing from all my safety warnings, remember this: PVC shrapnel will not show up on an X-Ray. The doctors will have an extremely hard time picking all the plastic shards out of your body if something goes wrong. You have been warned.
organicengines adds a helpful reminder to keep your PVC out of direct sunlight, because the light will degrade the plastic and make it brittle. Be careful!
Yes, this could be a dangerous project. I'm not a lawyer, but I'd like to say that I'm absolving myself of all responsibility for your actions. If you decided to build one of these cannons, by all means have fun, but please be safe. I'm not responsible for anything except for fun. If any of the Instructables administrators or moderators feel this project is too dangerous for this website, I will gladly remove it as soon as possible.
Another safety point: This is a cannon, which is a type of gun. Guns are meant to be dangerous, they would be pointless if they weren't. If you have no experience handling guns, please be very careful with this project, and do yourself a favor and take a firearms safety course. It will be a good experience even if you never shoot a gun again. If you have experience with guns, please be careful regardless. When I went through hunter's safety, they taught us three important rules of firearms safety:
1. Treat every gun as if it is loaded, so always point it in a safe direction. This means not at you, your dog, or anybody standing around. Point it up in the air, or down at the ground.
2. Always keep your finger off the trigger (valve switch) until ready to fire. Enough said.
3. Know your target and what's beyond. Plywood, fences, and garage walls are not strong enough to stop some of the things you can shoot with this cannon. I've put clean holes through both walls of an empty 55 gallon steel drum, so know what's behind your target. Try to not shoot this cannon in residential areas, or use really low pressures and soft projectiles. Soft projectiles include rolled-up, wet socks (more on this later), water, and loose snow.
That being said, this cannon can provide hours of entertainment, as well as a bit of education, in projectile physics and the ideal gas law.