Potato gun

A potato gun or Potato cannon is a type of cannon that uses combustion or air pressure to fire a projectile. They are called potato guns because potatoes are a commonly used projectile.
There are two main types of potato guns. Combustion guns use a flammable fuel and igniter to move an object. Another type uses compressed air. Potato guns are usually built of PVC or ABS (a type of plastic) piping. If using air to power your potato cannon you need around 50 to 70 psi to shoot a potato.
Combustion potato guns
Parts
Combustion potato guns have 4 parts. These are the barrel, combustion chamber, a cap, and an igniter.
Barrel: The barrel is the part of the gun which the projectile is fired from. It is usually a smaller diameter than the combustion chamber.
Combustion chamber: This is the part where the fuel is put. The igniter is also in the combustion chamber.
Igniter - A device which set the flammable fuel on fire. Electric grill igniters and magnesium latern lighters are often used. This could be called a trigger.
Cap - The cap is connected to the combustion chamber. It is removed to add fuel and replaced when the gun is fired.
Operation
the operator will put a projectile into the barrel. A ramrod is often used to push it all the way down to the end of the barrel. The projectile must fit tightly into the barrel so that the pressure of the expoding fuel is strong enough to move it. This is why potatoes are often used. With a large potato, the unneeded part of potato would be cut off when the potato is forced into the barrel. This creates a projectile that is almost the exact size as the inside of the barrel.
The cap is removed and a flammable fuel is put in the combustion chamber. Aerosol hairspray is a popular fuel because it ignites easily. Once fuel is added, the cap is replaced.
The cannon is pointed at its target and the igniter triggered. This causes the fuel to burn. The rapidly expanding gasses force the projectile out of the end of the barrel.
A potato cannon can move a 2" diameter potato projectile over 100 yards at a speed of 40 mph. This makes potato guns dangerous.
Air powered potato guns
Parts
A source of compressed air (such as air compressor)
Barrel:
Air reservoir - This is where compressed air is held before being released. It serves the same job as the combustion chamber in the other version.
Release - a type of valve. It is placed between the air reservoir and the barrel. This could be called a trigger.
Operation
the operator will put a projectile into the barrel. A ramrod is often used to push it all the way down to the end of the barrel. The projectile must fit tightly into the barrel so that the pressure of the air is strong enough to move it.
The air reservoir is filled by connecting it to a compressed air source. The release be closed at this time.
the cannon is aimed and the release is pulled. This causes the compressed air to rush into the barrel. The high pressure forces the projectile out of the end of the barrel.
A air powered potato cannon is much stronger than a combustion cannon. A combustion potato gun will launch projectiles at a pressure of about 10-20 psi. Air powered guns often use pressures over 100 psi.
Other websites
Instructables.com
Howstuffworks.com
Weapons