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A Spud Gun may refer to one of two things, a classic childs toy which fires small pieces of potato with a small volume of low air pressure or a large pipe based cannon which uses higher air pressure or explosive combustion to launch much larger chunks or even whole potatoes for recreation. Other larger, less conventional ammunition has also been successfully launched.
The die cast childrens toy may also be known as a potato gun, citrus cannon or lemon shooter.
The larger pipe based spud gun may also be known as a potato cannon, potato gun, produce accelerator or air cannon. They are made of no fixed materials but PVC, copper, or ABS pipes and associated fittings are commonly used in their construction.
This article is primarily about the larger pipe based spud gun.
The popularity of spudguns is mainly due to their low cost and simple construction, a simple cannon being easily made with little prior knowledge from common pipe and parts from a DIY store.
Spudguns vary in size greatly, from launchers shorter than 6" (15cm) with .25" (.5 cm) barrels to rather immobile devices taller - and larger around - than a man, if one decides that the truck-mounted pneumatic launchers of the american pumpkin-chucking competitions count. However, most launchers are sized as to be easily man-portable and fireable, and therefore are shorter than 10' (~3 meters) and not so powerful as to be uncontrollable.
All spudguns propel their projectile down their bore using compressed gas in the same manner as a firearm (although at a pressure nearly several hundred times lower). Most enthusiasts divide them into three general categories by the means in which this gas is provided.
By the release of compressed gas (normally air) through a valve; such a launcher is typically referred to as a pneumatic launcher, and its power is limited primarily by the pressure of the air supply, be that from a compressor, manual pump or bottled gas.
By the combustion of a gaseous fuel-air mixture; this is generally referred to as a combustion launcher, and its pressure is limited primarily by the energy density of the fuel-air mixture (less than 100 psi (7 bar) with all safe fuels).
By the combustion of a "pre-pressurized" fuel-air mixture; this is called a hybrid launcher, and yields higher pressures than that of a normal combustion spudgun, limited only by the construction of the launcher (generally a few hundred psi).
Combustion powered spudguns can be the least complex to build. They have four basic elements:
In order to fire, the operator loads a projectile into the barrel, adds fuel to the combustion chamber (for example aerosols or propane), and triggers the ignition source (e.g. a piezoelectric BBQ ignitor). The fuel should then ignite, create hot expanding gases, and force the projectile out of the barrel. Distances vary greatly depending on many factors, including the type of fuel used, the efficiency of the fuel/air ratio, the size of the launcher, and the flight characteristics of the projectile, but 100 to 200 meters is fairly common distance, with some cannons even exceeding 500 meters of range.
Advanced combustion launchers may include metered propane injection to ensure proper fuelling, chamber fans to mix the fuel with the air and accelerate venting of the chamber after firing, multiple spark gaps (spark strips) to decrease combustion time, and high voltage ignition sources (flyback circuits, stun guns, camera flashes, etc).
Despite their ease of construction, combustion launchers are usually less powerful than their pneumatic or hybrid counterparts.