A Homemade 1.25kJ Coilgun

This homemade coilgun is very impressive.

There has been a debate on AR15.com about how powerful this gun is. My guess would be a 800 grain slug traveling at 100 ft/second and generating just under 18 ft/lbs (24 joules) of muzzle energy. This is guesstimate fits in with the inventor’s estimate of 1-3% efficiency (1250kj * 0.03 = 37.5 joules).

This gun would still have recoil. Newton’s third law of motion cannot be circumvented by the use of magnets. Please correct me if I am wrong (UPDATE: I am!), but I think the recoil would be spread out during the time the projectile passes through the “barrel” (magnetic coils), unlike a firearm where the recoil occurs after the bullet exits the barrel. A marksman would need to counteract the recoil whilst aiming the gun, much like a machine gunner. This would have a horrible effect on accuracy, but the felt recoil would probably be more comfortable.

[ Many thanks to Ben and jdun1911 for emailing me the link. ]

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • SpudGun

    Makes the rifle I made out of rubber bands and clothes pegs look somewhat primitive.

    I’m assuming the inventor used such a large slug to get the maximum amount of electricity / magnetic acceleration onto the projectile. Would like to see a Mark 2 version using a smaller and more aerodynamic bullet.

    As for speculating on recoil, it’s difficult for me to make an accurate guess (mainly because I learned all of my physics from Star Trek), but I would imagine that it would be negligible compared to a firearm due to the lack of rearward moving bolt mass and gas pressures.

  • Josh

    The recoil would be long and drawn out compaired to a regular firearm. The Gun would push back the whole lenth of the barrel. And peaking just as the round leaves it. The navy is working on bigger versions of this to mount on ships.

  • A firearm’s recoil begins when the firing pin is released. At that time some matter is moving forward, therefore other matter has to move backwards – recoil. It becomes noticeable once the propellant ignites.

  • Kasper

    If the projectile accelerated by the magnets produce recoil, so does a projectile accelerated by heated gas. The ordinary guns must suffer as much by this recoil as does the space gun here.

    The reason you feel that the recoil is after the bullet leaving the barrel, is I think is because of the gas escaping faster/more violently when the bullet is gone.

    Of course moving parts in the regular gun going backwards will lessen the recoil felt by the shooter, which could lead to a lesser recoil than that by the space gun.

    My point: The recoil created by a bullet, has nothing to do with how the bullet is accelerated, electromagnetism or heated gas. Other things will make you think the recoil is different, but is has nothing to do with what technology producing the force.

  • Clean technology, CSI wouldn’t love it.

    But it is NOT firearm under most jurisdictions. He-he-he. Gunbanners suxx.

  • Ivan

    As magnetic coils push the slug, so slug push the magnetic coils. Newton still right. Coilgun can have less recoil because of absence of gunpowder gas(~20-40% of recoil impulse).
    But 3% efficiency is too small to do some more seriously… and interesting.

  • Falcon500

    I want one. That’s all i’m saying.

  • Jake

    As an engineering major, Steve, I can assure you that you are correct with your prediction of the recoil. It’s hard to say, however, whether or not the felt recoil would be greater than a traditional semi-automatic firearm, given that this coil gun has no visible method to reduce recoil.
    In any electromagnetic weapon, recoil energy would probably be more difficult to reduce or recover. The propulsion of the projectile is (usually) provided by magnets along the entire length of the barrel, rather than the gas of the combustion.

  • Don

    I don’t think recoil only begins when the bullet exits the barrel…

    The force accelerating the mass of the bullet must create an opposing force in order for that to happen, and the mass of the bullet is being accelerated inside the barrel.

    Imagine a hypothetical gun where the projectile was more massive than the gun+shooter. When you shot it, you and the gun would go flying downrange and the projectile would move very little.

    Or imagine a “Shake Weight”


    This (hilariously absurd) thing creates recoil without mass leaving the object.


  • Glen

    Recoil doesn’t occur until the bullet exits the barrel? Wouldn’t that mean that, for a few thousandths of a second, guns tell Newton to piss off? I doubt it. As I understand it, reaction occurs the instant action occurs. As soon as the propellant begins to push the bullet in one direction, the bullet is going to begin pushing in the other direction; ie. against the expanding gases which, in turn, push against the rifle. Now, that energy is going to take some time to overcome the inertia of the rifle so that felt recoil may be delayed. But recoil begins as soon as the firing pin begins travelling toward the primer.

    As far as the coil gun is concerned; of course it has recoil. It’s going to BE low due to the low energy of the projectile. Its going to FEEL low due to the high inertia of the weapon. Also, due to the relatively long transfer of energy, that recoil impulse is going to take a relatively long amount of time – thus making the recoil feel lighter.

    That’s my take on it, anyway.

  • Anton

    Meh, just get a PCP. :p

    A buddy of mine is building a plasma rifle. Not sure what it’s called but you get huge amounts of energy from that.

  • impressive gun. I hope they share the building sheet…altho I doubt they would.

    As for the recoil…err…why would you think it happens on a normal gun only after the bullet exit the barrel? As newton law implies in order for the static bullet(as it is in chamber at the moment before the trigger pulled) to move there must be an opposite force. So, if there is no barrel on a normal gun, the recoil happens only at the instant of firing, and if there is the gas will propel the bullet all the way until it leaves barrel, hence recoil will still happen all the way when bullet travels inside the gun. So I reckon this is exact the same condition as on a coil gun.
    I guess a machine gun needs more counteracting force simply becuz the recoil is non-stop as a result of continuous firing.

  • Oh I forgot to mention, speaking of recoil, I seem to remember Metal Storm claiming that their rate of fire could be so high that a three-burst could happen before the shooter feels the recoil? And some other gun like G11 or AK 74 could have similar effect.
    I think for a single shot the bullet travels so fast that on no matter what gun the shooter must feel the recoil long after the bullet is out. For a coil gun to be practical the bullet must have similar,if not considerably higher velocity-which is kind of a point of any electrical powered kinetic weapon, for it could generate muzzle velocity far greater than chemical powered firearm could theoretically produce. So really I wouldn’t think the magnetic acceleration process would affect the accuracy in any way different from how gunpowder does.

  • JC

    Very cool! He should test it on the Box O Truth.


  • DaveR

    for the record: recoil in a firearm begins the INSTANT the bullet begins moving

    a coil gun and a firearm of equal mass, shooting the same projectile @ the same velocity will have the same “recoil”

  • curtis

    i don’t think it would be diffrent recoil wise as a fire arm starts recoiling the moment the bullet starts moving, however the inertia of the gun at rest usully isn’t overcome untill after the bullet exits teh barrel.

  • kranren

    The recoil for this would be like a bolt action rifle (or other gun with a manual action) firing a slug like that; in fact, firing a slug the same as a rifle (same mass, same velocity), it would have less recoil, since the wasted propellant gasses of the rifle are added to the recoil. The recoil would be spread out along the slugs path down the barrel, but technically this is also true for the rifle – Newton’s laws and all that.

  • William

    The energy felt during the recoil of a firing gun starts the moment the powder starts to burn; it pushes just as hard against the back of the bullet as it does against the web of the case at the same exact time, however due to the fact that recoil only causes a heavy gun that’s connected to an even heavier person to move back anywhere from fractions of an inch to a few inches (and because it takes your nervous system a while to realize what happened), the bullet is long since out of the barrel and down range. But initial movement of both projectile and firearm in opposite directions happens at the same moment.

    Spudgun-You are partly correct in the gas assumption. Up to a third of the felt recoil in weapon (depending on the cartridge), especially in a bolt-action, is the weight of the gas. In it’s powder form there is a substantial mass to it, and it’s gas form loses none of that mass, so when it all gets blasted out the muzzle it adds to the recoil already felt by the projectile.

    I made a coil gun in my engineering physics class, but not nearly this impressive. It used a straw for the barrel, a bb for the projectile, some very thing gauge insulated wire, and a disposable camera circuit. I could shoot the bb about 10 feet. No perceived recoil 😉

  • Jim

    Very low power indeed. A small slingshot would do better at those distances.

  • JKEverett

    Recoil is going to be proportional to the force of the projectile leaving the barrel (see Newton’s 3rd). The fact that there is no mass moving rearward in the gun itself is going to increase perceived recoil, I think. I would imagine it would be much like firing a bolt-action weapon, or, on a larger scale, a cannon without a recoil dissipation system. If it was able to automatically cycle in some fashion that would, potentially, decrease recoil.

    The moving bolt systems on modern firearms dissipates perceived recoil because the force of the fired projectile is gradually introduced, via springs, to the whole of the weapon system, and ultimately into your shoulder or what have you.

    Or at least that’s how I’ve come to understand it…

  • Nanban Jim

    One way to find out more about it: Box O Truth.

  • ZK

    The recoil of a firearm does not occur after the bullet has left the barrel. The recoil begins as soon as the bullet overcomes friction and begins to accelerate.

  • It will have recoil. Any projectile firing weapon has recoil from the moment there’s a directional force. Recoil in a firearm starts as soon as the powder ignites, it just takes a bit for the force in the opposite direction to start the gun moving.

  • Dick Pilz

    As soon as the projectile or bullet start to move one way, the firearm starts to move the other way. The main mass of the firearm actually start to move forward as the bolt mass moves rearward. This is cancelled out (and then some) when the rearward movement of the bolt mass is halted.

    Every time some component changes position, mass, or speed, there is a corresponding reaction.

    I remember firing a M40 106mm recoiless rifle with a worn breech bushing. It jumped FORWARD with each shot because of the increase in back blast.

  • CMathews

    I made a coil gun for a science fair in junior high, albeit not as nice or as powerful as this. Mine would launch small tacking nails about 20ft. As far as recoil, the way I understood everything behind it is that it would be more spread put but only stronger towards the end of the barrel as the overall velocity increased. No doubt this guy has skill. But coil guns are terribly inefficient and will be for a while. Maybe one day we can have a concealed carry coil gun, but not now.

  • spudfiles

    “unlike a firearm where the recoil occurs after the bullet exits the barrel”

    Newton’s third law implies that any launcher starts to recoil the moment the projectile starts to move.

    If I remember well the Germans had theorised about such a weapon during the second world war, as a large calibre far reaching anti-aircraft gun, but gave up when it transpired that each weapon would need it’s own power station to keep it in action.

  • Rob

    Actually, firearm recoil begins at exactly the moment the projectile begins accelerating in the barrel- this is dictated by conservation of momentum. An easy way to think about this is if you’re standing on a raft in a lake, and start walking in one direction, the raft starts moving the opposite way as soon as you start walking- not after you fall off the edge of the raft into the water.

    Now you can do all sorts of things to reduce the instantaneous acceleration you feel from the recoil, by having moving parts in the gun that recoil first (like in all semiautomatic actions), but eventually all of the momentum is transferred to the shooter one way or the other. (With minor exceptions like guns with compensators that actually fire a reaction mass backwards away from the shooter).

    You may in fact find that firearms have greater recoil than a corresponding coilgun because you are accelerating not only the bullet, but also the exploding gases that shoot out of the muzzle of the firearm after the bullet leaves, in some cases faster than the actual bullet. If those gases are faster than the bullet upon exit, they will have a greater recoil effect per mass than the bullet itself.

    An interesting aside regarding conservation of momentum, is that momentum cannot be “re-vectored”. (energy can be re-vectored, but momentum is the effector of recoil)The Kriss “recoil-re-vectoring” system is a scam, to put it as nicely as possible. It works no better than having a linear spring and reaction mass (slide) slowing down the effect of recoil.

  • Komrad

    Yes, the recoil would be spread out over the entire time the bullet is moving down the barrel. But with any coil or rail gun that can propel slugs at a weaponized speed, the amount of time it would be spread out over would be negligible.
    If we say a practical weaponized speed is around 1000fps, then even with a 24″ barrel, the amount of time the slug is in the barrel is still less than 1/100th of a second.

  • peter

    nice that would be cool to have and just to shoot stuff

  • Matt Litman

    Please correct me if I am wrong, but I think the recoil would be spread out during the time the projectile passes through the “barrel” (magnetic coils), unlike a firearm where the recoil occurs after the bullet exits the barrel.

    I believe that recoil in a traditional firearm does not “wait” until after the bullet has exited the barrel. It takes a force to accelerate the bullet in the barrel, and the reaction force (which must occur simultaneously) is the recoil. There will also be an additional component to the recoil as the bullet exits the barrel due to the gasses jetting out the muzzle.

  • Gary

    A small point .. recoil occurs at the instant the projectile begins to move out of the case of the cartridge. It doesn’t wait for the projectile to leave the barrel.

    • Gentlemen, Thanks for the corrections

  • SpudGun

    I think we’re splitting hairs on the ‘recoil’ front, too much Physics and not enough Perception. I would imagine that any weapon system that has a more ‘gradual’ acceleration of projectiles, would feel much milder to the user then an instantaneous bang.

    I’ve never fired an AT-4, an RPG or a Stinger, but as they are propelling comparatively heavy projectiles at speeds faster then most bullets, the recoil should be enough to knock most people on their kiesters. However, that doesn’t happen.

    There are probably other factors that come into play such as correct bracing, disappation of force, the mass of the shooter, etc. which help in the perception of recoil. Just a thought.

    • SpudGun, those weapons are recoiless rifles. They are simply tubes used to aim a rocket as it accelerates

  • Cymond

    Most of you are correct; the recoil will be similar regardless of propulsion, except for the influence of the burning gunpowder. Slow motion videos (like this one http://www.youtube.com/watch?v=BW93WQ98s-I) show guns recoiling heavily after the bullet leaves. This happens for several reasons. First, the gun is heavier than the bullet so it accelerates (and “decelerates”) more slowly. This causes a delay before the recoil kicks in. Second, the escaping hot gas is allowed to accelerate freely and add to the recoil.

    The rearward moving bolt in a traditional semi-auto does reduce recoil a little. Watch some videos of gas blowback BB pistols if you don’t believe it. The pistol actually recoils forward and down when the slide goes back.

    “For a coil gun to be practical the bullet must have similar,if not considerably higher velocity-which is kind of a point of any electrical powered kinetic weapon, for it could generate muzzle velocity far greater than chemical powered firearm could theoretically produce.” A coil gun does not need to fire at a higher velocity if it fires a projectile of the same size and weight as a traditional weapon. Additionally, there is a limit to how fast a projectile can go because of air resistance. The faster it goes, the more dramatically the air slows it down. Militaries use depleted uranium because increasing the projectile mass is more practical than increasing the velocity.

  • Oswald Bastable

    Think black powder. Slower acceleration means a different feel to recoil- more shove than slap.

  • There’s a difference between recoilless guns and rockets. A recoilless gun (like the AT4) fires just like a normal gun: the projectile contains no propellant and reaches maximum velocity as it leaves the barrel. The only difference is that a large part of the propellant gasses (or a counterweight of some kind, depending on the design) blasts out the back to balance the recoil.

    With a rocket like the Stinger there is no cartridge and the launcher is simply a tube holding the rocket, which carries its propellant with it and carries on accelerating after it’s left the tube.

    The Russian RPG-7 actually combines both systems: a small recoilless cartridge gets it started, then a rocket sustainer takes over once the projectile has left the weapon.

    A final comment on the recoil issue: yes, the propellant gases escaping from the muzzle can provide 10-50% of the recoil energy (the greater the weight of propellant relative to the projectile weight, the higher the percentage it generates). But if a muzzle brake is fitted to deflect those gases rearwards, this will actually reduce the recoil.

    • Tony, ah right, I has the RPG-7 in mind.

  • GeoffH

    What you really need to do is analyze the acceleration of the projectile as it travels through the barrel of each weapon. If both projectiles are the same mass and reach the same velocity in the same length barrel, they could still produce different effective recoil depending on the acceleration curve. I’m not sure how you would measure the acceleration of a projectile in the barrel of a conventional firearm, but you could probably model it fairly well if you had enough information at hand. The coilgun would be much easier to model than the conventional firearm.

  • Cymond I think I know what you mean. For a projectile to have greater kinetic energy it could be either case of increasing its speed or increasing its mass. But with electromagnetic weapon we are talking about significant increase of speed.

    Wiki says a railgun’s projectile operates at a speed of 3500m/s or higher, nearly 4 times of a NATO 5.56. Of course whether a gun should increase its projectile’s speed or mass is subjected to practical measures available but electromagnetic weapons do have the benefit of increasing muzzle speed beyond conventional gun could possibly achieve :”the limits of gas expansion prohibit launching an unassisted projectile to velocities greater than about 1.5 km/s and ranges of more than 50 miles [80 km] from a practical conventional gun system.” from Adams, LCDR David (2003). Naval Railguns Are Revolutionary –a cited reference in wiki’s railgun article. Not exactly saying a coilgun, but I hope you know what I mean.

    Using depleted uranium to increase the projectile’s mass to utmost possible is a practice to increase the projectile’s kinetic energy when no other mean is possible or feasible. I didn’t check this material’s density nor did I do any math related to it, and please correct me if I’m worng but I’m guessing it’d be impressive if this approach would double the power of the weapon. While using electromagnetic force it’s easily 4 times more powerful we’re starting with. So I think with such ability easily available I hardly would think electromagnetic will still have the need to put dangerous heavy metal into its projectile just to get a relatively marginal power boost, not to mention the negative effect that increased mass brings, e.g. greater bullet drop. But again, of course no practical coil/rail gun is built and put to actual use at this moment and putting a rifle makes bullets go 4 times faster than m4 wouldn’t happen any time soon, so arguing which mean that this kind of weapon would use to increase its power is somewhat meaningless- but what I’m saying is it’s bullets should go faster, simply because there is no otherwise reason to use them as weapon at the first place.

  • big bear

    tony the at4 is not a recoilless rifle it like the stinger is a shoulder fired rocket and the recoil pushes you forward instead of to the rear due to the blast of the rocket pushing forward

  • @big bear: the AT4 doesn’t use a rocket. See: http://www.janes.com/articles/Janes-Infantry-Weapons/Saab-Bofors-Dynamics-AT4-84-mm-light-anti-armour-weapon-Sweden.html

    “The AT4 HEAT round consists of a cartridge-case assembly, similar to that used with 84 mm Carl Gustaf ammunition and a HEAT projectile. The main parts of the HEAT projectile are the fin assembly, the base fuze, the stand-off cap and the HMX/TNT hollow charge with a special liner. The fins automatically extend after the projectile has left the muzzle to stabilise the round in flight.”

  • Cymond

    Ok, I’m struggling to explain this concept, but increasing velocity gives diminishing returns. If anyone can explain this better, please feel free to clarify. Yes, I realize this is all moot until someone actually builds a working railgun capable of extreme velocities. I once heard a military expert talking about depleted uranium in artillery. He explained that increasing the velocity requires huge increases of propellant but yields minimal results because the extra speed drops off quickly when the bullet hits the air. Hence, he claimed that increasing mass is the only practical solution.

    [edit] I used a ballistics calculator to compare a 55 grain, 5.56mm bullet at 1250 feet per second and 5000 fps (the calculator is limited to 5000 fps). Quadruling the velocity produced the same velocity/energy at 700 yards as the 1250 fps shot at the muzzle. The more I looked at the results, the more I realized my critical error.

    I still disagree with drastically increasing velocity via burning propellant. We’d need to consider increased pressure issues, barrel length, weight, noise, recoil (from the projectile and the increased hot gas), and probably a lot of other factors I’m overlooking. Yes, the extra velocity does drop off quickly. However, my efficiency argument is probably moot when the weapon is stationary or ship-based artillery and the ‘propellant’ is cheap electric energy provided by a warship’s nuclear reactor. Less noise, no propellant based recoil, no pressure problems, no cost to transport extra powder, none of the traditional problems of using bigger ammo. Electric-powered artillery would also give the operator the ability to fine-tune the velocity and trajectory.

    Steve, please forgive the long post and thank you for a great conversation starter.

    • Cymond, long comments are encouraged 😉

  • Cymond, you are basically correct that (other things being equal and within reason) increasing projectile mass does more to extend the effective range than increasing velocity. To give a simple example in small arms ammunition, the US military recently improved the long-range performance of the .300WM in sniper rifles by increasing the bullet weight from 190 to 220 grains and dropping the muzzle velocity. The reason? The longer, heavier bullet has a better drag coefficient and loses velocity and energy more slowly, so at long range it is actually travelling faster (as well as hitting harder) than the lighter bullet.

    A more exotic example is 120mm tank gun APFSDS ammo, in which the long, narrow “dart” has such a good drag coefficient that it slows down only very gradually. If you could tilt the gun up to around 55 degrees elevation, the dart would fly to over 120,000 metres.

    As you suggest, the standard pros and cons of projectile weight vs velocity which apply to conventional ammunition might not be quite the same with railguns, but I don’t know enough about their performance parameters to say.

  • Kelli

    Hey I was wondering how much this gun weighs.


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