The piston inside my spudgun, is probably the most time consuming component to engineer. I have built two pistons, and each one has had many refinements.
I made the piston out of two "layers" of PVC, the inside is made of two 2" sections of 3" schedule 40. The outer layer is made with a 4" piece of 4" drain pipe. I cut out a small lengthwise section of the pipe, put it inside a 4" piece of schedule 40, put it in a bowl of water, and then microwaved it, until it was bent into the appropriate shape. I then glued the drain pipe to the two inner 3" pieces, but I put a large homemade washer in between the two as part of the check valve.
A thick metal plate with five holes drilled in it goes at the rear end, and a thicker piece of metal about 3"x1/2" goes in the front. I drilled a hole in a rubber stopper that fits in the end of my pipe, but a 5.5" bolt through it, then the steel plate, then two springs, then a washer to seal the homemade washer in between the 3" sections of the piston, and then the bolt goes through the metal plate in the back with holes in it, and then you put the nut on.
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This flash video illustrates how the piston works. Right click on it and select 'play' to advance.