Overall length is about 8 feet. The white PVC pipe is 1-1/2 inch Schedule 40 PVC. The black "chamber" is 6 inch diameter ABS joint section with screw in caps..
The ABS Chamber is connected with the barrel using a regular 6" to 1-1/2" adaptor... and glued with PVC cement.
The striker is a Coleman lantern striker. Drill a hole through the ABS, that matches the dimension of the shaft of the striker, disassemble it, and mount it like shown in the pics....
The bevel on the end of the barrel is very important, Ramius says, to ensure the clean coring of the potato when you ram them in the end of the barrel. Use the little new red potatos that are slightly larger that the barrel, and just press them in... they will form fit... tamp down to just short of the chamber with a pole (marked for the length so you don't push it all the way through)
Propellant: We use Naptha (Lighter Fluid).... Other people have used other stuff, even Aqua Net hair spray, but the lighter fluid works best. The lighter fluid needs to evaporate inside to fire, but you can't really put in too much. Only the amount of vapor the air will hold will actually evaporate, and the excess will remain liquid and not hurt anything. Pour in a good squirt of it. You'll figure it out in time. On cold days, it is hard to get it to evaporate... let it warm in the sun and turn it slowly for a few moments, then point it in a safe direction and fire.
One safety mod that Ramius says he would like to do is to wrap the whole chamber in duct tape, because if the ABS were ever to blow, it shatters like glass, and the duct tape might hold down the shrapnel.
In close, Ramius and I both want to emphasize that these are dangerous weapons.... treat them like a gun. I have seen a potato shatter a cement block at close range. It would be lethal if you hit someone with it, and the potatos will go a couple hundred yards. The beach is the best place to fire them.