How a Quick Valve cannon works

To first understand how a piston-valve cannon works you must know how a 'quick exhaust valve' cannon works. Here is a simple diagram of a diaphragm gun. There are several important parts to notice. From left to right...

The ball valve on the back allows you to release pressure. The small intake valve can be hooked to an air compressor or a
bike pump. The separator between chambers is a stiff (yet flexible) rubber sheet. It acts as a diaphragm. The main pressure chamber should be several times the diameter of your barrel. The barrel extends inward all the way to the diaphragm.

Click to get a bigger version.

This animation of the diaphragm shows how it works.

Gun is loaded, with no pressure. The release valve is shut.
Pressure is pumped in. The two air chambers equalize, because the pressure leaks around the diaphragm.

Pressure stays in. The barrel is a "low-pressure" zone, and sucks the diaphragm toward it. This keeps it sealed.
Open the release valve, and the first chamber depressurizes. This sucks the diaphragm away from the barrel, so the air can escape, firing the bullet.
Gun is empty and depressurized.

This design and picture was taken from http://www2.csn.net/~bsimon/pngun.htm. The picture is a miniature of the actual cannon (not to scale).The diaphragm is somewhat flexible and it will bend it's outer edges forward from the pressure and allow the air to fill the outer chamber. At the same time this forward pressure is sealing off the end of the barrel preventing air from escaping up the barrel. Pulling the blowgun out of the charging hole allows the air behind the diaphragm to escape. At this point all the pressure stored in the outer chamber bends the diaphragm in the opposite direction sealing off on the outer edge of the cleanout plug. The diaphragm continues to bend under the pressure and unseals the barrel end allowing the stored air to escape up the barrel behind the spud. This method far outperforms the over-under 'sprinkler valve cannon' above because the diaphragm allows more volume of air to enter the barrel than the sprinkler valve.

A 'piston exhaust valve' cannon (P.A.C. 5) is the same as the quick exhaust cannon above except for one exception. The diaphragm is changed to a piston assembly. This creates a larger opening for the air to get from the air chamber to the barrel, thereby pushing the spud out the barrel much farther than before. A piston can actually move (not just flex) 2 inches or more.

When the air is charged in the cannon, the piston is sealed against the barrel, much like the cannon above. When there is a sudden drop of pressure behind the piston, the piston is sucked back, letting the air enter the barrel.

The piston that I made, is constructed out of a 3" coupler made to fit into 4" pipe. Here is a little drawing of how it is constructed: