These are my favorite air cannon sites. There's plenty to be learned by visiting the following sites:

Air Cannons of the Internet - Exhaust valve and pneumatic cannons!

This site has some great designs. The CAD drawings are a huge plus if you want to make your own exhaust valve cannon. His rev. 3 piston is fairly easy to make!

Andy's Aerodynamic Inventions - Great pneumatic and exhaust valve cannons!

My very first pneumatic air cannon was designed after his PAC4 cannon! In fact, I built my exhaust valve cannon when he posted his item list for his PAC5. Andy's site is full of inventions! He has combustion, pneumatic and exhaust valve cannons. Not only that but he has designs/pictures of a silencer, breech loader, projectiles, etc. Tons of stuff. A must visit!

Master Blaster Pneumatic Cannons - Bad ass exhaust valve cannons!

Home of the Big Red. Wish I could build that bad boy! This site has lots of picture's and MPEG's of assembly and usage of his cannons. You must check out the MPEG's he has: PC monitors, a dead tree, notebook computer, doghouse, etc all destroyed! Jaw dropping fun!

Spud Gun Technology Center - Combustion and pneumatic cannons!

Ed has designed the creme de la creme of combustion cannons! The SP9004 is simply a work of art! S.G.T.C. has a wealth of information concerning spud cannons. You must check it out!