The following are pictures and component descriptions of the Paintball air cannon that I built and have used very successfully. This article is for informational purposes only. As far as I know this is the first Paintball cannon of its kind. It is basically an air powered potato cannon with a few twists added to make it more suitable for recreational paintball.
 
 

This system uses a floating barrel, that is to say that the barrel is slid forward through a set of collars to release a short section of barrel from between a set of couplers. This short section of barrel is then replaced with another section that has been pre-packed with paintballs in a cardboard sabot. The barrel is then slid back, closing the couplers on the new loaded barrel section. A clamp that reaches from one coupler to the other is then latched and locked to secure the preload and floating barrel in place. The whole process takes 10 to 15 seconds to perform.

The firing mechanism for this cannon consists of a RainBird electric sprinkler system valve, two 9-volt batteries and a momentary pushbutton (the RainBird valve has been commonly used in airpowered paintball and potato canons). The valve goes between the low-pressure reservoir and the barrel assembly. The power supply consists of two 9-volt batteries wired in series to produce 18-volts (the RainBird valve is rated for 24 volts but works just fine with 18). It's triggered by a momentary pushbutton switch mounted in a gutted out plastic grip assembly from a Spyder paintball gun. Pushing the button provides 18 volts to the valve, which opens allowing the pressurized Co2 to pass from the reservoir to the barrel.

Note: I use a Co2 tank with an on/off valve. Though technically you can leave the Co2 tank on full time, I recommend that the low pressure chamber not be filled until you're ready to fire the gun. This minimizes the risk of regulator failure or the possibility of falling on the gun and breaking open the pressurized chamber. THINK SAFETY FIRST!

The cannon itself is made mostly of PVC pipe. The low-pressure reservoir is 2-inch PVC, the barrel and pre-loads are 1.5-inch PVC. I found that the smaller barrel achieved higher, more consistent velocities. Connecting the reservoir to the barrel is a standard .75-inch, U-turn RainBird sprinkler valve. The barrel slips through two couplers that have been reamed out to allow to barrel to move somewhat freely forward. Attached to the back of the barrel and the outlet of the valve are couplers that have been reamed just enough to allow the pre-loaded barrel piece to fit snugly between the two when the barrel is pulled back. The couplers are held fast by a spring-loaded clamp that stretches between the barrel and the valve. The input from Co2 tank and the reservoir is made of small brass pipes and fittings common to any good hardware store.

Over the past couple of years, many commercial paintball fields have outlawed the use of multi-ball air cannons such as the BECC, and have instead opted to go with soft projectiles such as nerf rockets. Despite it's 1.5 inch inside barrel diameter, the BECC is able to use commonly found finned nerf footballs. The footballs are a tight fit in the pre-load chambers of the BECC, but with about 120 psi the little footballs fire amazingly straight.
 

There are no drawn plans or parts list for the BECC. The gun was pieced together in a trial and error process, and is still subject to further modification. For larger detailed pictures of the latest version of the BECC, click on any of the following three pictures.
 

This cannon is designed to be used against armored vehicles and bunkers in scenario games and not (for obvious reasons) against other players unless used in a long range, mortar fire fashion. I hope this information is useful.
Play safe and have fun.