A Solenoid Sprinkler Valve is used in irrigation and lawn systems.  You can obtain one from a landscaping Company or maybe your local hardware. 

It is like a trap door that opens with a charge from a battery.  Shown below I have posted a diagram to show how it works.  You have to hook it up to two 9 Volt batteries and a
Momentary Switch that can be found at Radio Shack.

Well this part isn't all that simple for all people.  You are going to need to wire a circuit to make a connection with the valve, battery leads, and a momentary switch.  So make sure you have all those parts ready.  The arrows should be facing your barrel.

I apologize for such a crappy drawing.  But with this drawing you should see exactly how the circuit is put together.  Also with the pictures you'll see how it looks on the cannon. 

By using the Pneumatic Add-On Page, the Loading the Chamber Page and this one you'll know exactly how this baby does its wonders.