It's a
myth that you can put too much
propellant in a potato cannon and have it
blow up. The optimal ratio of propellant to air, called the
stoichiometric raio, is when there is exactly enough
oxygen for all the propellant to
combust. If you have less propellant in the
combustion chamber, you'll get decreased power becuse there's less propellant to burn, if you have more, you'll lose power because there's not enough oxygen to burn all of it.
Pressure-rated
PVC plastic is probably the best allround material for building spudguns. It's relatively lightweight, guaranteed to hold
pressure (the ratings also have generous
safety margins) and a correctly done solvent weld is just as strong as the rest of the
pipe.