Well not that simple... but anyway, Here they are, the instructions to building your own air gun. (these pics are from the building of my most succesful gun) it is a 1/2inch PVC pipe, you can get a 10 foot PVC for like 3$ at home depot. also you got to get the PVC fittings, they are like 0.30 each, once you got the 10ft 1/2inch PVC, cut it up with a saw and make it the way i did or wathever you want.
Make sure your valve is AIR tite before you get one. the rest of instructions on vid.
Disclaimer: I take no responsibility for your actions. DO NOT shoot crap at people or animals or yourself IT CAN KILL very easily. shoot away from anything you dont want broken. have fun!
Just remembered, PVC produces POISONUS fumes if it is burned so please don't put it in a fire.