Parts needed:
-Combustion Chamber
A combustion chamber should be about 2 gallons in volume. Any container can be used that will withstand the pressure of the explosion. A strong container such as the pressure sprayer we used, or PVC piping of a large diameter.
-Barrel
The barrel should be between 3-5 feel in length depending on the combustion chamber size. For diameter, we prefer between 1-3 inches. However, for larger projectiles, a larger diameter will be needed. The barrel should be SECURELY and airproofly to the chamber. PVC or ABS piping should be used. Make sure the barrel will allow easy passage of the projectile. File the end of the barrel down to allow easy "adoption" of the projectile to the barrel width.
-Ignitor
The ignitor should be a gas grill type. Take a piece of wire, and attach it to the grounding wire of the ignitor. Make a small space between the bottom of the ignitor and the wire for a spark to jump. This will ignite the propellant. Also, electric alternatives can be used. A voltage sufficient to jump a gap of 1/2" should be enough.
-Propellant
Hair spray and WD-40 is the most common propellant, however, people that are more than mere amateurs may wish to use stronger propellants, such as butane or propane. Remember when filling the chamber, do not put too much in as to drive all O2 out of the chamber.
-Projectile
Apples, Potatoes, Eggs, Tennis Balls, wrap small objects in paper-towels for shotgun like blasts.