Potato Cannons, commonly refered to as spudguns, potato guns, etc, are typically made of two pipes, one being a reaction chamber and one being the barrel. The potato goes down the barrel, and the chamber is filled with Aqua Net hairspray or WD-40 or something like that, and it is ignited by a barbecue ignition.
THERE IS A BETTER WAY, FOLKS!
I will now tell the story of my friend Richard and I.
Its our senior year of high school, in I town I commonly refer to as Pleasantville, or Hell, and we're both on our school's ASB. We come up with the idea of shooting candy and t-shirts into the bleachers at the upcoming Renaissance assembly. The advisor liked it, and told us to spend whatever we needed to get it done.
We set to work. Richard gets ahold of a small 3000 PSI rated fire extinguisher for real cheap, and we set out to Ace Hardware to buy parts. A nice four foot length of PVC did nicely for the barrel, and that two foot high pressure air hose looks nice also... A couple of bushings here and there, and we're out of the store.
We are lucky enough that every sprinkler manufacturer in the world is located in our town, so we head down and buy a $90 solenoid valve rated up to 250 PSI. wow...
Radio Shack is next. Some nine volt batteries, wires, buttons, lights, and a project case and we leave. Back to Richard's garage.
If you haven't already figured it out, we mounted this fire extinguisher case onto my back, put the solenoid in line with a pressure gague, attached it all together with the electronics, and attached a barrel with a nice round button on the end.
Taking an air compressor, we hooked its hose to the fire extinguisher, filled the thing with compressed air, and had a very nice projectile weapon in our hands.
Sure, we used it at school at like 40 PSI, shooting candy and t-shirts into the crowd... but that thing shot a potato three times as far as ANY WD-40 cannon I've EVER seen.
A couple of neat little tricks to try.. Load the barrel with some powdered and mixed thick mashed potato solution, sit in the back of a truck, have someone ring a doorbell as you aim.. They open, you fire, mashed potatoes everywhere! Drive away as quickly as possible. Also, get some of the same pvc pipe you used to make the barrel, and cut off about four or five inches, fill it with water and freeze it. That thing will go through a brick wall.
Have fun, don't get hurt, and know that air pressure kicks ass!