The principle of the golf ball cannon is simple. I used a  standard electric air compressor to pressurise an airtight reservoir to around 100 psi (700 kPa or 7 bar equivalent). This high pressure holds a freely moveable piston against the breech which effectively seals the reservoir exit.
The area on the front of the piston that is exposed is reduced because the front is up against the breech. Thus, the total pressure on the piston here is relatively less than the air pressure which pushes on the rear and holds the piston firmly against the breech, preventing the air from escaping. The rear of the piston is open to the atmosphere via a quick-acting valve.
Opening this second valve precipitously reduces the pressure behind the piston suddenly and it recoils, immediately allowing the pressurised contents of the airtight reservoir to escape along the barrel, accelerating the golf ball.
For obvious safety reasons it is important to use high-pressure PVC components, obtainable at any plumbing supply outlet. Rated to in excess of 800 psi, these will give you a reasonable safety margin. Guttering downspouts and the like are not suitable. There are no particular tricks to making the body of the cannon, but do allow at least 24 hours for the PVC adhesive to dry before you test it with pressure.

Read more in the April/May 2010 issue of The Shed