Steel Pipe
Steel pipe comes in many flavours (Galvanized, stainless, etc), and is a good choice for the construction of a spudgun.
Advantages and disadvantages of steel pipe:
Advantages:
-Readily avaliable at many hardware stores.
-Rated for pressure (Full details can be found at Here)
-Threaded fittings
-Cheap, relative to copper. It is also avaliable in larger diameters.
-Durable
-Will not melt (but it will heat up!).
Disadvantages:
-Heavy
-Length of pipe cannot be changed, if threaded fittings are used.
-More difficult to drill into/tap threads than PVC.