Fail Of The Week: [Chris] Vs. The Gorn

This week, [Chris] tips the scales but ultimately fails. He’s on the road, hacking through the Great White North and improvising from a poorly-lit echo chamber that happens to have a vise.

Knowing nothing about firearms (do you believe that?), he decided to build a BB cannon out of pure scrap. Several kinds of sparks fly, starting with a Hitachi drill-as-lathe and ending with a tiny cupcake sparkler. [Chris] proceeds to bore out some redi-rod by eyeballing it and offers helpful tips for course correction should you attempt same. Having centered the cavity, he drills out a tiny hole for a fuse.

His first fuse is of the crushed up match head paste variety. It burns kind of slowly and does not launch the BB. Naturally, Plan B is to make napalm glue to adhere Pyrodex pistol powder to paper. As you might imagine, it worked quite well. The wadding was singed, but still no joy. After packing her full of propellant, it still didn’t explode and merely burned out the blowhole. So, what gives? Insufficient barrel length? Should have used bamboo instead of redi-rod? Didn’t want it badly enough? Give us your fodder below.

2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which runs every Thursday. Help keep the fun rolling by writing about your past failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

30 thoughts on “Fail Of The Week: [Chris] Vs. The Gorn

  1. Ideas:

    1) Touch hole should probably be funnel shaped. See:

    2) Instead of a fuse in the hole, you might consider a pan. (Small reservoir on top of the touch hole holding powder). Let the powder communicate the combustion through the hole. (Or pack the powder into the hole with the pan on top. It seems like the fuse is just ending up getting in the way and snuffing out your ignition.

    1. like everything, the thrust vectors to the point of least resistance. If your fuse hole isn’t significantly smaller than the barrel, then any packing you did will serve to direct the expanded gasses out the hole from which it’s ignited.

  2. The reason that the fuze hole is only about 1% the diameter of the cannon bore, is to allow the pressure to build up behind the load. I would guess that the fuze hole is too large.

  3. First off there is a difference between black powder and nitrocellulose propellant. What you want is black powder. Second off the bore diameter to touch hole diameter ratio is too close. Gasses will pick the easiest route. Why push wadding and projectile out of the bore when it’s far easier to just burn out through the touch hole.

    1. If [Chris] tries gun/black powder, he should be informed of how it works.

      If ignited when unconfined, it just burns. Quite vigorously, but does not explode. An example is the pyrotechnic “flash pot” you might have seen at music shows, that sends flames feet into the air. (Though this can be done with propane also.) I’ve thrown gunpowder into a stainless steel bowl and ignited it to demonstrate this; it’s “safe” because the bowl has a wide open top, so as not to confine the reaction. Real flash pots use a tube to direct the flames better. But if you use too narrow and high a tube, especially if combined with a finely powdered gunpowder that burns faster…

      The gunpowder DETONATES instead. That’s an explosion. I’ve read of a small band that made their own flash pots, made this mistake, and a member was literally blown off the stage (but fortunately not seriously harmed). Denotation occurs when confinement causes the pressure to rise fast and hard enough, that it causes the temperature of the gunpowder to rise to the point where it essentially all ignites simultaneously. And that’s what you want for a gun. To a point at least, you don’t want such a vigorous detonation that it blows your barrel apart.

      I’m not going to say that [Chris] shouldn’t experiment with explosives or homemade guns. But I will give some advice. Always READ FIRST. Pack your head with all the information you can get, prior to doing anything where a mistake may cause injury. If you just learned something from me, or someone else here, I’m glad we could help; but that means it’s time to do some more reading on your own as well.

      1. Traditional black powder with a potassium nitrate oxidizer does not detonate (detonation is defined by the shock wave propagating faster than the speed of sound). Potassium chlorate or perchlorate variants might.

        With such a small powder reservoir, an electric igniter might work better than a fuse/pan and can be made without a large hole, but either way the result will be extremely variable with the grain size of the powder and any tamping of it.

        Also, don’t tamp unconventional bp loads.

  4. Back in the day…. (old fart alert).. we used to build these with a steel pen refil and small Chinese fire crackers. Never actually managed to blow myself up or sustain major sharpnel wounds with one, but it could embed a bb in your leg… I think I can probably find the scar if I look carefully. These days with all the elfin safety that surrounds us, chinese firecrackers are banned … probably metal pen refils are too, but they only give me wax crayons these days, so I don’t know… (ItsThatIdiotAgain goes back to rocking on the porch and dribbling in his porrige).

  5. I’ve played around with some FFFFg black powder and I wouldn’t really recommend trying to do anything with it other than maybe use it to ignite the main charge. It is very volatile and the one thing it can do well is pass ignition through a very tiny touch hole.

    That said, I don’t think I would try spending my time making a gun in this manner. If you’re wrong about the amount of pressure produced or if you’re wrong about the barrel’s ability to hold that pressure, it’ll go off like a bomb. The same thing could happen if your projectile gets unexpectedly stuck in the barrel. Everything I did with black powder was fairly low pressure and I built a blast cabinet with a lexan window to contain the (potentially flying) debris if something went wrong.

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