A combustion-based spud gun has three basic components: an ignition source, a fuel chamber and a barrel. When firing a spud gun, the user first pushes a potato past the barrel knife and all the way down the barrel. Once in place, the potato acts as a seal at the end of the fuel chamber. The user then removes the end cap from the chamber and fills it with a flammable substance, such as hair spray or aerosol deodorant. After securely replacing the end cap, the user allows the fuel and air to mix inside the chamber and then presses the ignition.
An actual igniter placed inside the chamber would be prone to damage from heat, fire and fuel. That's why many spud guns use two metal points within the chamber to create the spark. Current from the ignition device -- often a grill starter or stun gun -- travels into the chamber, and a spark jumps from one point to the other, just like a spark plug in a car.
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The spark ignites the fuel, causing an explosion. Pressure builds rapidly behind the potato, which is wedged tightly in the barrel. When the pressure in the barrel overcomes the resistance from the potato, the expanding gas forces the potato down the length of the barrel and into the air.
Now, let's look at how pneumatic spud guns create the pressure necessary to fire a potato.