pretty much cant beat co2. as long as there is liquid in your tank you get about 1100 psi. all you need is a fat needle valve like a garden nozzle .shown here. get all the brass fittings and PVC at home depot and take the tank adapter off a paint ball gun and there you go a potato gun. or t-shirt cannon. or mega super soaker depending on what size PVC you use. guarantied not to catch fire and provide you with hours of fun.
