Propellants
There are several types of propellant you can use. there's the gas form, like butane, and the solid state form, like gunpowder.
Before using any new types of propellants you must find out what burn rate they have. Generally for spud guns you need a low burn rate, like deodorant. Because if you use a high burn rate fuel it may make the gun explode.
Also the fuel must not condense easily otherwise the gun will not fire.
At the moment I am using diesel easy start that has a slow but powerful burn rate. but be warned, it is very powerful and is to be used in small amounts.
If you have any better fuels E-mail me