Well here it is, this is an older pic but it still works, the only change from this pic and the current cannon is that I have put some duct tape around the PVC bonds and am planing on spray painting it. The compression chamber is over a foot and a half long at 2" diameter PVC to push the spud or what ever you can out the cannon. The barrel is a little more than a foot long. This was originaly a prototype with all 1" PVC pipe but I cut it all off and attached a few PVC sizers and made the barrel and the chamber 2" while still using the 1" design. I guess you can consider this to be physics as it uses air power. It can be made into one using explosions but that is too much work for me at the moment.