A pneumatic Gun operates under 2 main ideas. (1) It must use compressed air. (2) must have a release valve of some sort. If you are familiar with spudguns read this page to figure out just how high that you can pressurize your gun.  
There are 4 main ways that you can release the air

(1) A ball valve. Quick definition= a quarter turn valve that is manually operated. Available in plastic or metal.

Pros

  • No hassle with electrical wires
  • No batteries needed

Cons

  • Very slow (causes tremendous drag)
  • Have to be periodically replaced if you don't break them
  • Difficult to operate quickly 

(2) A Solenoid Valve. Quick definition= an electrically released sprinkler valve.

Pros

  • Much faster
  • Push-Button Switch activated
  • Hardly ever need to be replaced; very durable

Cons

  • Use electrical wires
  • Use batteries
  • Wires can't get wet what so ever (shortages)

 

(3) Mega Air Dumping Valve.  I have no idea how these things work. They are obtainable through  The Spud Gun Technology Center 

From what I've heard, these monsters have gotten some very good reports from customers because of their speed and size of the release mechanism

(4) You can use a solenoid valve in a piston construction

This is the construction I am using on my howitzer. Very effective.  Soon there will be several diagrams on my howitzer concerning this process.