why wouldn't people want to help

I know this is a stupid question, but how to you make a piston valve that is easy to build, but able to wiithstand ~100 psi? I want to make a gun like the Mega Launcher II on spud tech able to shoot tennis ball over 200 yds.

I am on a budget, so i cannot use metal, but I do have an extensive supply of PVC. The object is to shoot the spud the farthest, but like i said, the barrel can only be 2.5 inches in diam. and the whole gun can only be 6 ft long (sorry, earlier i didn't say the whole gun 6 ft) I am really just looki...

This is a school project and the teacher does not us to want to stand close to it, but we are allowed to use propane or aerosols

I need to make a spud gun with a 2.5 inch barrel, max of 6ft long, remote firing system(stand far away) and need to shoot it as far as possible. What kind of material do i need to use for the gun and propane or aerosol?

Thanks,
Patrick