Made out of PVC Plumbing stuff...
The spark comes from the striker from a Coleman lantern. Drill a hole and install the lighter inside with the striker sticking out and seal it.
Stuff the potato all the way down with a tamper (little round red potatos work best, must fit tight)
Open the chamber, (Ramius's has an screw-on cap) and pour in the accellerant. Lighter fluid is what we use most often.
Close it up, turn it around slowly to disperse and evaporate the accelerant, point it in a safe direction and flick the starter. (the potatos go 100 yards or more depending on the quality of the load)
Here is Ramius and his potato cannon, photo taken last weekend at his birthday!
And since Webshots sucks, here is the pic URL... if/when the picture breaks... paste this in your browser:
http://community.webshots.com/storage/1/v4/1/97/93/43619793ooDkGb_ph.jpg