Introduction: Air Powered Cannon
An air powered cannon consists of three parts; a compression chamber, a release valve and a barrel. I used gravity PVC pipe because I am an idiot. Use pressure pipe for the compression chamber, not just because of safety, it's also the only thing that will join to a quality solenoid valve. It's suggested to use a bicycle valve that's threaded and tap a hole into the compression chamber but if your budget is small, get an old tire and cut the valve out, glue it in and caulk around it.
Solenoid valves are the recommended release mechanism because the air is instantly released whereas a ball valve trickles then releases. Solenoid valves don't need to be modified, all that's needed is a momentary switch with a 9 Volt battery wired in series.
The barrel can be long or short but you do lose energy using a long barrel but gain accuracy and vice versa with a short barrel. As an added special feature you can make an enclosure for the release switch (like mine) and use the same pipe as a holder.
I recommend checking out (https://www.instructables.com/id/Spud-zooka/)
I vaguely followed this instructable (http://www.youtube.com/watch?v=7Z-L4GliAts&feature=c4-overview-vl&list=PLbDYIZ0o1HPJScdEdZ-39n6Am4HyJnmve) by the King Of Random
I also found when firing it's good to use wadding behind whatever your shooting out and you can also make a very cool explosion simulator. For more details follow this link (https://www.instructables.com/id/How-to-Make-an-Air-Powered-Explosion-Simulator/)
Note- THIS IS NOT A TOY, IT'S A WEAPON, NEVER AIM AT ANYTHING YOU DON'T WANT TO KILL OR DESTROY