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.