Spud gun
Spud guns, potato cannons, or produce accelerators are all recreational launching devices usually made of polyvinyl chloride (PVC) water pipe. There are two different means to launch projectiles from a spudgun. The first uses the energy stored in a compressed gas and the second, more popular method, uses the energy stored in combustible fuel vapors. The object of both of these varieties being to propel "non-destructive" projectiles in the direction of a designated (safe) target for fun. Common projectiles include: mandarins, potatoes, and tennis balls.
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Combustion launchers
Combustion powered spudguns have four basic elements:
- Fuel system
- Combustion chamber
- Ignition source
- Barrel
In order to fire, the operator loads a projectile (usually a potato) into the barrel, adds fuel to the combustion chamber (for example hairspray 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 the type of fuel used and the "idealness" of the fuel/air ratio; 100 metres is a common distance.
High tech combustion launchers may now include metered propane injection, chamber fans, multiple spark gaps (spark strips), and high voltage ignition sources (flyback circuits, stun guns, camera flashes, etc). Combustion launchers are mechanically simple, but can usually outdistance a pneumatic spudgun, just by sheer power.
Pneumatic launchers
Pneumatic launchers consist of five basic elements:
- Compressed gas storage reservoir (chamber for short)
- Filling valve (bicycle schrader valves, quick-disconnects, etc.)
- Main valve (ball valves, sprinkler valves, piston valves, and quick-exhaust valves are the most common)
- Barrel
- Pressure gauge (for safety)
In order to fire, the operator loads a projectile into the barrel, fills the chamber with a compressed gas (air compressors, hand pumps, and regulated bottled gas are common filling methods), and activates the main valve thus dumping the air in the chamber to the barrel.
The expansion of the compressed gas propels the projectile down the barrel. The barrel length determines the amount of energy transferred to the projectile and ultimately the velocity of the projectile.
High performance launchers use techniques such as modifying valves for faster opening times and maximizing the amount of airflow through the valve.
There are also designs for spud guns which involve the use of a bottle bomb, a.k.a "works bombs" these are less controllable than pneumatics, which makes them less reliable, but they can work just as well with a big enough bottle.
To make a works bomb, one must get a barrel (usually PVC piping) and seal one end of the barrel, so that no air must escape. This can be done by placing metal screws through the end of the barrel and pouring concrete in. The next step is to get some toilet cleaner called "The Works", which is 20% HCl (the strongest one can find at the average grocery store). Next,pour some of the toilet cleaner into an empty water bottle (can be any size, but the bigger the bottle, the further the projectile flies). After about an inch of the bottle is filled with cleaner, put some aluminum foil in the bottle (the amount of foil determines the time required to burst) and screw the cap on. The HCl reacts with Al to produce AluminumChlorohydrate and Hydrogen gas. The Hydrogen builds pressure in the bottle until the bottle burts, suddenly releasing hydrogen gas and launching the projectile hundreds of feet.
Works Bomb powered spud guns are not as powerful as combustion or pneumatics, but they are certainly easier and cheaper to construct.
- A good alternate projectile for a works bomb powered gun is a softball. Softballs fit perfectly inside of 4" PVC piping, and a 1 liter bottle is a perfect fit for the piping as well. Works bomb powered softball guns can launch a softball over 100 yards and over 150 ft in the air.
Performance issues
Some tricks to improve the performance of a pneumatic launcher include:
- Oiling the barrel (the less friction and resistance, the faster the potato will shoot)
- "Oiling" the main valve. It is bad to use oil, as it will cause deterioration, but household soap can be used (valve opens quicker, more force to push potato out)
- Obtaining higher pressure piping and a higher pressure pump. Most PVC pipes have a maximum pressure of 280 lbf/in² (1.9 MPa). Higher pressure launchers can theoretically shoot potatoes farther.
- Be sure to file down the end of the barrel so that when you load a potato it is trimmed by the sharpened edges to the correct size.
Safety issues
Improperly designed or assembled spud guns can be unsafe. Using proper assembly techniques and correct materials (pressure rated pipe and fittings) is essential. The use of a spud gun which has developed visible structural defects (ie. cracks) or has not been constructed properly is dangerous and can result in serious injury. Safety is paramount while operating a spudgun; treat them with the same respect you would a real gun.
Legal issues
The United States Bureau of Alcohol, Tobacco, and Firearms does not classify spud guns as firearms. However, certain states and most cities have outlawed them. Check with your local law agency before shooting a spudgun to determine their legality. Never fire a spudgun within city limits, there are too many dangers involved by doing so. If authorities tell you not to use a spudgun in a certain area, it is in your best interest to follow their directions.
Vandalism that involves the use of spudguns contributes to the illegality of the hobby, so if you know someone who intends to use a spudgun for such purposes, alert authorities immediately. The use of dangerous, explosive and/or flammable projectiles is illegal and not condoned by the spud gunning community.
All spud guns have been classified as illegal in Western Australia.
All combustion spud guns are considered firearms in Germany.
Pneumatic spud guns are illegal to possess if you are under 16 in New York.
In the UK, pneumatic spud guns with projectile energy greater than 16 joules (12 ft·lbf) are classified as section 1 firearms and require a licence.
In Walnut Creek, California, potato cannons are considered legal.