hey, i just got a 200 psi air compressor and i want to build a pneumatic cannon...since my home depot wont sell pipe by the foot, i have to buy the whole 10 foot pipe

...since im not willing to spend more than $50 on my pneumatic cannon, im going to use the 8 feet of 4'' and 1 1/2'' pipe left over from my combustion cannon. since i dont know much about pneumatics, can someone give me plans for making a pneumatic cannon that uses 4'' and 1 1/2'' pipe? Im hope the valves and all of the connectors won't cost over $50.
Ty,
nabu92