The Design shown here is one of the most common designs for a potato cannon.
You will need:
A 24 inch piece of 3" dia Sch 40 PVC pipe
A 3 feet piece of 2 " dia Sch 40 PVC pipe
A 3" to 2" reducer
A clean out cap and adapter
Gas grill ignitor (Sideburner Type or Hotshot™ Type), spark plug , and wires
Start by gluing the pipes together . First glue the reducer to one end of the 3 inch pipe.
Now glue the clean out cap adapter into the other end. Screw the cleanout cap into the
adapter. This forms the combustion chamber.
Now glue the 2" pipe into the reducer . Now you have the barrel.
Now the hard part, wireing the fireing device. On the end of the ignitor, there is a metal tab.
Strip the wire and twist or solder on to the tab. This is your positive wire. Now locate the tab on the side of the ignitor and solder another piece of wire to the side tab. This will be the ground wire. Connect the other end of the ground wire to the base of the spark plug. Connect the Positive wire to the end of the spark plug. Give the ignitor a press. The sparkplug should spark. (see picture for details.)
Drill a hole in the side of chamber for the spark plug the same size as the threaded part of the plug. Screw the spark plug in.
Tape the grill ignitor someplace where you will be able to push it. I put mine's on the reducer so it is like a trigger .
Wait overnight for the glue to dry or your gun will fall apart when fired.
(Trust me, I had this happen before.)
Now the fun part, Load a potato into the gun through the barrel.Push it 1/2 way down with a broom stick.
Unscrew the cleanout and spray some fuel into the chamber. Close the cap.
Aim at what you want to shoot. Call out "FIRE IN THE HOLE!" and press the ignitor.
If the gun fires, you now have a working spud cannon.