When Airsoft gets boring, build a coil gun!

airsoft_coilgun

Here at Hackaday, we’re all about repurposing old items you no longer use. Reader [Liquider] wrote in to share his latest creation, a coil gun built from an old Airsoft pistol.

He removed a handful of components from the pistol and installed a 800 uF/300V capacitor inside the grip. A small storage compartment was added under the barrel, which houses the AA battery he uses to drive the circuit. A modified reloading mechanism makes it easy to drop a metal projectile right in front of the coil before firing.

Once the pistol is charged up, a switch installed behind the trigger discharges the cap, creating a magnetic pulse that accelerates the metal projectile forward. [Liquider] estimates that the kinetic energy produced by the coil is 0.1 Joules, which fires of the slug at a reasonable speed.

Continue reading to see a quick video demo of the pistol in action.

Comments

  1. Addidis says:

    Best post ever ! and on my list of things to build. Dam that looks sexy.

  2. steve says:

    Nice. I am pleasantly surprised by the high energy. I guess what he is using is basically an old photo-flash to power the thing?

  3. meatman says:

    A++ add a second coil sequenced and a second cap and you will double the speed of the projectile. I did somethign similar with a plastic PVC pipe and 6 coils spaced logarymythicly and a nail board.

    If you struck the power aacrossthe nail board just right you could put a 3/4″ ball bearing through a brick wall.

    • heine says:

      I think you meant to say “logarithmically”, but I like your spelling better.

    • andar_b says:

      My understanding says that a spherical projectile is much less efficient in a coilgun than a cylindrical one. Just a note.

      • adamhixon says:

        Why would a cylinder be better?

      • Dax says:

        Incidentally, the larger the projectile and the lower the exit velocity, accompanied by a larger inductance in the coil, will result in greater energy transfer efficiencies. Smaller projectiles and higher exit velocities have the opposite effect. That’s a fundamental limitation of the switched reluctance coil gun, because small projectiles get saturated fairly quickly and to apply more force quickly you need a deeper potential energy hole – i.e. a stronger magnetic field.

        It is possible to boost the efficiency considerably by changing the projectile into a non-ferromagnetic material such as copper or aluminium, and inducing a strong eddy current in it, which will repel the accelerating coil and push it off instead of pulling it in.

        Other than that, the only way to make a coilgun work with meaningful energy is to use the coil as a solenoid that drives a piston that shoots pellets with air pressure.

      • draeath says:

        Could you not “capture” this solenoid bolt simply use it to propel the actual projectile? Done right you could even use another coil to brake the bolt to reduce wear on the capture mechanism.

      • Dax says:

        “Could you not “capture” this solenoid bolt simply use it to propel the actual projectile?”

        If you mean, why can’t we use the solenoid to kick the projectile directly, the issue is that you would then be better off simply shooting the solenoid bolt itself and we’re back to square one.

        The point of the suggestion was to use a solenoid that has a big metal plunger that moves relatively slowly over a short distance, because that makes the coil work efficiently.

        You then use this forceful but slow motion to squeeze gas through a narrow pipe like in an air gun to achieve greater velocity for the actual projectile.

    • Maave says:

      Got any pics of that? Sounds pretty nice.

    • steve says:

      [ ] you understood the relationship between speed and impulse / energy

      • Dax says:

        @”Why would a cylinder be better?”

        It’s a bit difficult to explain, but these types of coilguns work via the magnetic reluctance principle.

        In essense, the magnetic field in and around the coil experiences a sort of resistance and it has to spread out because the density of the field is limited. The projectile being ferromagnetic metal, has lesser magnetic reluctance than air, so a magnetic field going through it can be denser. Any such metal is a path of least resistance to the magnetic field, so metal is naturally “falling” in this potential energy hole in the ground.

        Should the magnet not be switched off, the piece of metal would oscillate and eventually settle in the middle of the coil.

        So the projectile’s shape and size determine the size of this potential energy hole. A small projectile will only occupy a small portion of the flux path and get less energy transferred to it, but a large projectile may simply weigh more and get less speed. There’s an optimum between the two, and the shape is roughly cylindrical at about 3/4 the lenght of the accelerator coil.

  4. Why not use a series of starter solenoids to build a linear motor? If I wasn’t shot by Jack Sapphire I would do it.

  5. thaddius says:

    I’m going to disagree with meatman on this, I don’t understand your energy calculations. Doubling the capacitance only doubles the energy, unless you’re getting astounding gains in efficiency from that second coil, you’re not going to double the velocity. On an unrelated note, I’d love to see that coilgun you built–I’ve yet to see a good, high energy build.

  6. o says:

    this is really cool. not sure I understand the circuit entirely though… the little box that says “out 300v in 1.5v” is a little charging circuit, right? and anyone know what that 2N6509 semiconductor is supposed to be doing?

  7. Roboman2444 says:

    Now if only this recharged faster and had a clip…

    Also, I want to see a railgun built into one of these things :P

  8. Nexxus says:

    That was impressive. It’s almost very simple to add a second capacitor and a circuit that would charge the second capacitor, since it’s a capacitor firing the projectile, you have both capacitors charged set a two stage trigger so when you squeeze the trigger this discharges the first capacitor creating the coil charge that fires the round out, reload and squeeze the trigger all through to the second stage which would then fire the the second round. Alternatively your trigger could lock at first the when squeezed again the moves to second stage which resets the trigger back to first stage readiness.

  9. same says:

    @steve: man 0.1J is nothing compared to those airsoft guns between 0.9 and 1.5J

    Peace

  10. James Glanville says:

    I really need to do something with the ~80J I can store in my flash capacitor bank… apart from weld screwdrivers together anyway.

  11. Brent says:

    For comparison, an entry-level .177 air pistol like the Crosman 1377 produces about 7 joules at the muzzle.

    A typical .22LR rifle will typically produce around 175 joules at the muzzle.

    Further, either of the above will be substantially more accurate out-of-the-box than a beginning shooter, which is to say, they’ll actually hit what you aim at.

  12. Kiwisaft says:

    lets combine this with the coin crusher they presented here a few days ago :)

  13. Volectorus says:

    it looks like he gutted a old disposable camera.

    AAA not AA.

  14. Havel says:

    Man, I’ve been wanting to build one of these things for the longest time. My compliments on the build. It looks pretty friggin’ awesome and seems like it works well. Since I’ve seen this I’ve hit the net, and started researching. One commenter said something that had wheels turning in my head. Thanks for posting!

  15. obsoehollerith says:

    Not bad! Looks like fun. My latest toy trials have me attempting to improve on the accuracy with collimator coils after the initial sequenced drive coils. Picky and all fiddly as hell, but hey? that’s where the fun is!

  16. wmehy says:

    nice ~

  17. mbear says:

    “Could you not “capture” this solenoid bolt simply use it to propel the actual projectile?”

    If you mean, why can’t we use the solenoid to kick the projectile directly, the issue is that you would then be better off simply shooting the solenoid bolt itself and we’re back to square one.

    The point of the suggestion was to use a solenoid that has a big metal plunger that moves relatively slowly over a short distance, because that makes the coil work efficiently.

    You then use this forceful but slow motion to squeeze gas through a narrow pipe like in an air gun to achieve greater velocity for the actual projectile.

    See Light Gas Gun and Combustion Light Gas Gun for more information.

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