High Tech Potato Cannon

20 thoughts on “High Tech Potato Cannon”

1. Great another small minded person creating a zip gun out of plastic. Wow, what a man

   Watch out where you expose those types of guns. Most states they are illegal.

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2. Do the big scary plastic guns bother you Schretter? Last time I checked, the ATF position on spud guns was that they are not considered a weapon unless used as such. Remember, potatoes don’t kill people….

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3. Wrong! Tell you what. Bring one to your local law enforcement office and see what happens. ;)

   ATF you crack me up. The laws are state level not federal.

   With a CHL you can carry about any weapon you want besides a switchblade and a zip gun. That is my point.

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4. Here is my other point:

   People keep adding more and more fuel and spark to discharge these weapons. Well when is too much too much????

   Guess they will find out when it blows up and maybe kill or hurt a few people.

   Me scared no. Scared of what will happen to someone when these things blow up yes…

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5. Your concern for your fellow man is touching, but I think that there are dangers in every field of endeavor, including the ones you personally find acceptable. Humanity is rife with enterprising idiots who final words may well have been “hold my beer and watch this.” People by nature are going to build bigger, badder and better everything, just because they can. Can you stop them by belittling their efforts? Not really, you just look like another safety nanny.. and I mean that with respect.

   BTW…. phone calls to the local law here in the backwaters of the Pacific NW. So far no one has knowledge of potato guns being illegal. Your mileage may vary.

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6. Okay…. return call from the local police. He says that they don’t have a local or State law & fall back on the ATF ruling, which I mentioned in my first post. It is illegal if used as a weapon.

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7. Some local areas do have laws against spud guns (class them as Firearms), but IME they are rare.

   “Great another small minded person creating a zip gun out of plastic. Wow, what a man ” You are trying to rile ppl up, aren’t you? Condesention rarely wins arguments, or friends.

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8. Using Low-tech spudgun to destroy high-tech laptop. http://www.flickr.com/photos/tma3/

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9. Please pardon the long winded article. I get excited about these things.

   I’m Pat, a new maker but old potato and pumpkin cannon maker here. I use PVC for smaller models because… it’s cheap. I agree that the sport/hobby/insanity can be dangerous, but that’s a strained CAN. The dangers I recognize include the projectile damaging property or maiming humans or wildlife as well as the cannon exploding leading to more personal damage than the projectile. I am no expert on legal issues so I’m no help there, but I do know the dangers of the materials used for these cannons.

   If I had my druthers I would make every cannon out of steel, weld all joints, design with a significant factor of safety, perform a hydrostatic pressure test on the cannon for peak pressure, and monitor stress levels at critical points in the vessel. I operate on a student’s budget so I can’t do that (right now I am concerned with buying groceries this semester!). But, as a student of engineering I can gain comfort understanding the material characteristics of PVC: it can easily be welded with solvent/cement, it is strong, and it is brittle. The last item is what concerns me the most when I craft air pressure or combustion devices with PVC.

   Tell your plans for the pipe to any PVC supplier who knows his product and he will be sure to make it very clear that PVC is never recommended for pressurized gases. PVC is dangerous because of its nature to fracture into many small shards like safety glass when it fails under stress. I spoke to a fireworks retailer once about building cheap launch tubes for aerial shells out of PVC and he shortly related a story about a shell exploding prematurely in one such tube. He said the doctors failed to remove every shard of PVC from his leg afterwards in part due to the fact that the PVC didn’t show up on their X-ray slides. So I’ve taken a different route, the new fireworks industry standard, of using HDPE tubes instead because of HDPE’s safer behavior at failure. Why not use HDPE for potato cannons? It can be more expensive, and it must be heat welded to assemble a cannon. I don’t have the equipment for heat welding.

   So, PVC is brittle and that is bad. What’s worse is the nature of pressurized gas. Pressurized gas acts like a big capacitor storing all the energy you can put into it. Let me use the metaphor of a brick and a spring. If I apply a force to a brick and instantaneously remove it nothing dramatic happens. If I apply the same force to a spring and instantaneously remove it the spring leaps from the surface it was restrained against showing that it stored a lot of energy. With regards to my metaphor, pressurized liquid is the brick and pressurized air is the spring. PVC is used for liquids for water supply and sewage piping because a liquid bears pressure without compressing (noticeably) so when the pipe breaks you have broken plastic and a stream of flowing liquid but now explosion. A broken PVC pressure vessel for gas is a pneumatic bomb.

   So, I said I use PVC because I can understand its characteristics; the following is what I know. The manufacturer provides the max operating pressure printed on the pipe. The solvent/cement manufacturer provides the max operating pressure the glue is rated for. Both these numbers have factors of safety incorporated in them. I compound that factor of safety by my own when I build. My PVC cement, the weakest rated component in my pumpkin cannon, says it is good for 180 psig (I assume the manufacturer has a factor of safety greater than 1 for this number, perhaps 2). So when I fire my pneumatic pumpkin cannon I operate at no more than 90 psi, 2 times the manufacturer’s factor of safety.

   Also, environment can weaken/damage PVC. I avoid exposing my pipe to extreme temperature and drastic changes in temperature because this can make it more brittle. UV light can weaken the pipe too. So a good measure is to keep the cannon covered in a basement or closet where the temperature is comfortable to humans year-round.

   Failure will also occur as a result of damage to the pressure vessel from careless storage, material fatigue from cyclic loading, scratching, as well as a result of poor construction habits.

   Ultimately the device’s maker and user are the only ones responsible for failure, and he or she runs a dangerous gambit. You might think after all I’ve written that I’m an opponent of these plastic potato guns or pumpkin cannons because I’ve said more about their dangers than their safety. Well, I’m a cautious miscreant. I’m into making loud noises, demonstrating large amounts of power, and doing things that make other people scratch their heads. But I like not to have my head blown off by my own creation. There you have it.

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10. Legal issues

    In many countries spud guns are outlawed or have restrictions on their use and may require licenses and certification of the gun.

    * United States: Not federally regulated (unless the ATF finds that it’s classified as something else, such as a destructive device), however legislation varies widely by state, county, and township. One would have to check with local authorities to find out if spud guns are legal in that area.
    o Pneumatic spud guns are illegal to possess if you are under 16 in New York.
    o In Glendale, Arizona and Phoenix, Arizona combustion spudguns are considered firearms.
    o In Madison, Wisconsin, spud guns are considered not to be firearms.
    o Combustion spud guns are considered firearms in the state of Texas
    * Australia: All combustion spud guns are considered firearms.
    * New Zealand: All combustion spud guns are considered firearms.
    * Germany: All combustion spud guns are considered firearms.
    * United Kingdom: pneumatic spud guns with projectile energy greater than 4 joules (3 ft·lbf) are classified as section 1 firearms and don’t require a license. In recent years, with the rise of spud gun use, there has often been much debate as to whether or not spud guns should actually be classed as Light Air Weapons. If spud guns were to be reclassified, then their maximum lawful projectile energy would be 12 ft/lbf, not 3.
    * Canada: No federal or provincial law regulates Spud Guns.

    Other notes:

    * Flaming, explosive, black powder, or living projectiles can often make a legal spud gun illegal in many jurisdictions.
    * Many heavily-populated areas have ordinances on projectiles and loud noise.
    * While combustion cannons may be legal in a given area, stun guns, commonly used for ignition, are illegal in many states.

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