a_simple_combustion_type_potato

This is how the components go together in a simple combustion type of potato gun. I used ABS parts for mine. You can use PVC if you prefer. I read somewhere that igniting hairspray only produces approx 14 - 15 psi behind the potato slug, so either PVC or ABS will do the job. I wouldn't use any of the real thin PVC though. Use at least Sched 40 (approx 1/4" thick). I think you will find that ABS is cheaper than PVC, and it has other properties that make it a good choice.

The igniter is from a barbeque and is fitted into a cap that has been drilled to fit (the igniter is modified slightly, so that when you're done, it looks like a car spark plug. A modified igniter is shown below). The igniter is held in place with a nut on the inside of the cap, and is sealed with silicone applied from the inside of the cap.

The end of the barrel is sharpened slightly so that it cuts the potato slug as the slug is being pounded into the barrel. For best range, make sure the volume of the barrel is 1.4 to 1.6 x the volume of the chamber.

Some people mistakenly believe that making the combustion chamber bigger will provide more range. As far as I know, this is a falsehood. If you consider the formula F = PA (where F = the force on the potato slug; P = the pressure in the chamber; and A = the surface area presented by the potato slug), you can see that in order to exert more force on the potato, you would need to increase either the pressure in the chamber or the area presented by the potato. Force is not a function of the volume of the chamber.

If you stay with this volume ratio, where the barrel = 1.4 - 1.6 x chamber volume, the barrel will look slightly too long (I think mine is around 40"), but this is what works best. You can easily measure the volume of the chamber and the barrel by filling with water, and pouring the water into a measuring jug to see what volume you have.

You can use 2" diameter tube for the barrel, but you'll find that it's much easier to find stuff to shoot if you make the barrel from 1 1/2" diameter. A 2" diameter barrel will deliver a harder punch. Some people use threaded adapters so that they can interchange barrels. A 2 1/2" barrel can shoot tennis balls and a 3" barrel will fit 20 oz. soda bottles. On mine, I can use apples from the tree in my yard, which is a bonus.

After making the barrel, drill through it near the point where it meets the chamber, so you can fit a 1/4" bolt through. Use a nylock nut ( nylon locking nut) on this bolt for safety reasons - it will never undo. The bolt will prevent the potato slug from falling into the chamber during loading. Seal the holes around the bolt with silicone. Use lots of solvent/glue and make sure everything is dry before testing.