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I'm trying to design a railgun to meet a very particular purpose: to destroy (i.e. render inoperable) modern man-portable firearms, such as pistols, rifles and grenade launchers, from a Glock pistol to a Barret M82 while minimising the harm done to the people carrying them.

These railguns will be small and carried by autonomous flying platforms controlled by highly capable artificial intelligences, which may be assumed to be able to fire their railguns on an ideal trajectory to hit the weapon but not the wielder. It may also be assumed that the firing platforms are able to hit their chosen targets effectively at will, even if moving, to within a CEP of 2.5mm at the railgun's effective range of up to 2 km. The projectiles are self-guided after firing, and can adjust for firing inaccuracies, air movement and target motion.

The question:

What combination of the projectile's dimensions, material and muzzle velocity would best allow the projectile being delivered on the ideal trajectory to render modern small arms inoperable, while minimising the potential for injuring the soldier carrying the weapon?

I don't necessarily need 'weapon is hit and explodes in a burst of fragments' unless the damage delivered is required to disable a larger, heavier, more robust weapon. The most robust target weapon should reliably be disabled by a single hit with a minimum of excess energy provided the target is hit within 7.5mm of the point at which it was aimed.

I would prefer that the person carrying the target weapon, whether wielding it or carrying it holstered or slung, not be injured any more than necessary. Bruising is acceptable at any time, bones may be broken or dislocated only if no other option exists, bleeding should be minimised, and loss of extremities or loss of life is unacceptable.

While this weapon may be used to cause human casualties, it is not its primary purpose.

I am not interested in debating the feasibility or otherwise of a railgun & slugs with these capabilities. Think of it as Clarketech if you need to. I am interested only in the dimensions, material and velocity range of the slugs needed to perform as specified.

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    $\begingroup$ What exactly is the question here? $\endgroup$
    – OT-64 SKOT
    Mar 3, 2022 at 12:43
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    $\begingroup$ Remember the weapon won't be operable against giant snakes; More seriously, even if the trajectory is perfect (either the bullet is near-instant, or the turret predicts exactly the movement), you'd need an extremely high-accuracy camera to perform it. I doubt most cameras are able to have a pixel out of a 2,5 mm target at 2km, even if their view angles are low. So careful with your aiming design :) $\endgroup$ Mar 3, 2022 at 13:05
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    $\begingroup$ I'd sooner see a grasshopper-sized drone latch onto the weapon and drill a hole in it. then it breaks the drill off. It targets the breach. $\endgroup$
    – Tony Ennis
    Mar 3, 2022 at 21:17
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    $\begingroup$ An edit to only change "man" to "human" does nothing to improve the question or its readability. While commendable to inclusivity with other sentient lifeforms, that single editorial change is superfluous. If there had been other edits made at same time...then those changes could be incorporated $\endgroup$ Mar 3, 2022 at 22:47
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    $\begingroup$ you mention having nano-scale bot capabilities. If this is accurate to 2.5mm, why not target the barrel of the weapon with a bot that can essentialy create welds where it shouldnt. sort of like the additive manufacturing version of the grasshopper bot idea. some weld or other obstruction down a barrel of a gun could block it and render it inpoerable. for larger guns, simply add more nanobots to put chunks of welded metal where there shouldnt be for that weapon. This process shouldnt damage wielder unless the weld is in a position to cause burns or the weapon is used while obstructed $\endgroup$ Mar 4, 2022 at 0:20

8 Answers 8

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Railguns seem spectacularly poorly suited to this task.

  • Railguns are unlikely to scale down to the projectile sizes required, especially while imparting enough kinetic energy to damage a weapon at 2 km.
  • Even a scaled down railgun would require a massive power supply, unlikely to fit on a flying platform.
  • Railgun projectiles almost have to be solid lumps of copper or aluminum to withstand the current densities involved in launch, or just to provide enough surface area to drive the current through. The electromagnetic, thermal, and acceleration environment is not conducive to complex electronics and guidance mechanisms.
  • The described precision is completely unrealistic from a ballistic projectile.
  • The described precision is even more unrealistic from a flying platform.
  • The described degree of control of the effect on the target is unrealistic.

This doesn't work. Even if your end goal is to disable enemy hand weapons via kinetic impacts, railguns aren't the way to get there. You want something like a micromissile with a captive bolt or tiny shaped charge, not a railgun.

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  • $\begingroup$ Could a small shaped charge disable a gun without seriously injuring a person using it, or would the explosion cause unavoidable injuries? $\endgroup$
    – Monty Wild
    Mar 3, 2022 at 14:05
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    $\begingroup$ @MontyWild Shaped charged are designed to punch through things. They eject a lot of highly energetic shrapnel out the other side. That's how they take out tanks. punch a tiny hole through the armor and let the shrapnel destroy whatever they collide with inside. $\endgroup$
    – sphennings
    Mar 3, 2022 at 15:02
  • $\begingroup$ @sphennings and they can easily be made to not go off if they hit off-target, adding to safety. Burns and shrapnel injuries seem likely, but realistically, you're only going to break the most fragile of guns without injury...never mind what happens if you make the weapon discharge in the attempt. If you really want safety, go for something like glue or noxious chemicals. $\endgroup$ Mar 3, 2022 at 15:15
  • $\begingroup$ @ChristopherJamesHuff According to OP system is 100% accurate. The danger is that if it hits the weapon it's generating a highly energetic cone of what used to be a functional weapon in the direction of whomever is carrying it. $\endgroup$
    – sphennings
    Mar 3, 2022 at 15:33
  • $\begingroup$ Another answer mentioned thermite, and a thermite micromissile seems even better than a shaped charge. I imagine it could slam into the gun and start burning hot enough to melt it, but slow enough for the person carrying it to drop the thing. Of course, it would cause a lot of fires, but I'm sure they have super-AI firefighting drones too $\endgroup$
    – Andy
    Mar 4, 2022 at 2:41
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This seems like a bad idea unless you're willing to do some serious handwaving.

If you're not wanting to just shoot the person holding the weapon, you're violating multiple fundamental rules of firearm safety. Firstly don't point your weapon at something you aren't willing to shoot. And always be aware of you line of fire and what lies behind your target.

Most modern weapons are designed to contain small explosions with enough energy to project a lethal projectile hundreds of yards. While they definitely can be shot and disabled this is a much more precise shot than that required to hit the combatant holding it.

While there are plenty of ways that a weapon can have it's performance affected by relatively low energy bumps and dings, a weapon that jams more often, is still a dangerous threat to anyone getting shot at.

If you miss you'll probably hit the combatant with a lethal projectile. If you hit the weapon there's a high risk of generating lethal shrapnel, or over penetrating.

If you're willing to accept those high odds of killing the combatant to take their weapon out of combat it will be strictly easier to just shoot them instead of their weapon.

If you had some sort of super AI that will not miss and is capable of predicting the movements of the combatant's weapon, and assume that overpenetration and shrapnel can somehow magically be mitigated, a rail gun can adjust the energy of the projectile depending on the targeted weapon, any small arms caliber is probably going to be good enough.

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  • $\begingroup$ The firing platforms are AI controlled, and the projectiles are self-guided, and will hit within 7.5 millimetres of the desired point 100% of the time, and within 2.5mm 50% of the time. The idea is not to eliminate overpenetration, but direct it to minimize harm to the soldier, and minimise energy transfer to the bulk of the targeted weapon. $\endgroup$
    – Monty Wild
    Mar 3, 2022 at 13:41
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    $\begingroup$ @MontyWild Railgun projectiles are unguided. That's why they need to go faster than things like missiles, which are guided. $\endgroup$
    – Daron
    Mar 3, 2022 at 13:45
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    $\begingroup$ @Daron These projectiles are guided. Even the US armed forces have been experimenting with guided railgun rounds. $\endgroup$
    – Monty Wild
    Mar 3, 2022 at 13:47
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    $\begingroup$ @MontyWild So a magic AI, with magic bullets? Rail guns can control the velocity of their projectile. Why don't you just make the launcher magic as well and always deliver just enough energy to disable the weapon, with 0 margin for error? $\endgroup$
    – sphennings
    Mar 3, 2022 at 13:47
  • $\begingroup$ That is probably possible to a degree... for reasons linked to the power system of these drones, shot energy would need to be limited to one of a few discrete settings. That still begs the question of how big a slug. $\endgroup$
    – Monty Wild
    Mar 3, 2022 at 13:51
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Slow Thermite glueballs.

I was pondering exactly this: how a drone could incapacitate a tank without killing the crew.

The projectile is quite slow and very sticky. It deforms on hitting, absorbing most of the kinetic energy and it sticks where it hits. Then the thermite starts up. The thermite melts some of the metal it is on but also distributes molten iron into the interior of weapon or vehicle.

If the glueball hits a person the person can wipe it off or disrobe to get rid of it. At worst the person will get a burn before wiping it off. If it hits a weapon the weapon will probably be dropped and the glueball can do its thing. If it hits an occupied vehicle above the occupants they will be able to move away from dripping molten metal. If some drips on a person he will be burned by it. For a vehicle in a nonoccupied area there are horizontal surfaces where gravity will help the glueball and it will be difficult to run out and get a ball off before it starts melting through. Molten iron added to electronics or engines or gun barrels will incapacitate them.

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    $\begingroup$ I was writing a very similar answer but without the termite stuff, thinking more of a "sticky expanding slime ball" that would render the inner mechanics of the gun useless or at least is able to absorb the energy of the gunpowder explosion $\endgroup$
    – Josh Part
    Mar 4, 2022 at 0:05
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    $\begingroup$ With a slow projectile you definitely need to have auto-guided, auto-aim bullets. Predictive aiming alone won't be enough. However, auto-guided bullets are presumed to be included in the drone package, so :)... $\endgroup$ Mar 4, 2022 at 11:19
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    $\begingroup$ @JoshPart something like a cyanoacrylate paintball came to mind as one possible approach. $\endgroup$ Mar 4, 2022 at 15:33
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Not possible to disable

First, what does "disable" mean? My take on this is that the firearm can no longer be discharged unless damaged/destroyed parts are replaced. Hence, blowing the trigger off a firearm or removing a few mm from the length of the firing pin is considered equivalent to punching holes in the chamber or turning the entire barrel into iron filings as far as "disabling" a firearm goes.

The problem here is that even though a CEP of 2.5 mm with all impacts within 7.5 mm of point of aim (POA) is implausibly good, it is not good enough to reliably disable a firearm without seriously or fatally injuring the user. The trigger is probably the most delicate part on most firearms, but it is only a couple of millimetres across, meaning that the majority of rounds fired at it would miss. Ditto for the rods (where applicable) connecting the trigger to the hammer, the hammer and sear etc. The biggest target for attack from all directions is the chamber, but even this is not terribly large.

Let's take a 9mm pistol as an example. 9mm rounds are straight (not necked) cartridges, so the internal diameter of the chamber is effectively 9 mm, giving a radius of 4.5 mm. Assuming a chamber thickness of 2 mm, this gives an external radius of 6.5 mm. As soon as a round targeting the chamber is more than about 4.6 mm off target it will be striking an inclined surface at an angle of 45 degrees, as a result deflecting at up to 90 degrees from its initial direction of travel. Given that human beings are less durable than the chambers of firearms (citation required) this means that a significant minority of rounds will be ricocheting unpredictably with ample energy to kill or maim the person carrying the firearm or others in the vicinity.

Not quite disabled

However, an alternative that does not quite disable a magazine-fed firearm is to distort the magazine well to prevent the insertion/removal of a magazine. The section of the receiver that a magazine is inserted into is definitely large enough that, assuming it can be engaged from the side, the target area is a basically flat surface with a diameter of > 15 mm, providing a guaranteed hit and no unpredictable ricochets. Punching a 4-5 mm hole with jagged edges through the top 15 mm of the magazine well will prevent insertion or removal of a magazine and, if there is a magazine currently in the firearm, will damage the round/s in the path of the projectile and prevent feeding.

This will not fully disable a firearm as the operator can still insert an individual round into the chamber and shoot. Turning all magazine-fed firearms into single-shot breech-loaders will significantly diminish the combat capabilities of modern armies though.

There is a caveat for even this option. Each railgun must be able to vary its muzzle velocity based on the range to the target and the type of firearm being engaged. If the railgun always fires with the same muzzle energy then a round that will punch through one side of the steel receiver of a 7.62 mm L1A1 at 2 km will drastically overpenetrate when targeting the hardened plastic receiver of a F88 Austeyr at the same range of 2 km or a L1A1 at 100 m. Given that a 5.56 mm SS109 is supposed to penetrate 3 mm of steel at 600 m range, I would guess that impact velocities for weapons to penetrate one side of the receiver without blowing clean out the other side will need to vary from 100-300 m/s, with the lower end for targeting receivers made of hardened plastic and the upper end for more hefty steel. Note that these impact velocities are quite low - if the projectiles are shot at hypervelocity railgun speeds then they will need to decelerate sharply just before impact in order to avoid overpenetration and resultant damage to personnel.

Projectile design 1 - only a relatively small calibre (5 mm?) projectile is required at the target in order to cause the appropriate damage to the receiver. The projectile needs to be made of a dense handwavium in order to:

  • survive the railgun launch forces
  • self-guide itself to the target
  • drastically re-shape itself to magically decelerate down to 100-300 m/s immediately before impact.

Projectile design 2 - if there is no reason for hypervelocity initial projectile speeds, then the rounds can be shot at much lower speeds, ranging from slightly over 100 m/s for point-blank shots at fragile receivers to maybe 1 km/s for shots at hardened targets. This has the further advantage that it requires far less handwavium for projectile self-guidance. The key difference with this shot profile is that long range shots will take multiple seconds to reach their target, during which time a person may move a body part into the path of the shot, beyond the ability of the projectile to steer around it. Therefore, the dense handwavium for these projectiles must be able to:

  • self-guide itself to the target
  • disintegrate some or all of itself into dust/filings that will become harmless within 10 metres. This allows a high mass projectile that will retain its velocity better over a long flight to become a low mass projectile just before it hits or allow the entire projectile to turn itself into harmless (within 10-20 m) "dustshot" if circumstances change so it is going to strike a person.
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Even if you assume that a flying railgun was capable of disabling a firearm (which Christopher James Huff very accurately refutes), such a projectile-based system would inherently violate your "minimize harm to the operator" principle.

As a simplified example, consider what is probably the most reliable and least complicated modern firearm option: the revolver. You have a couple of ways to disable a revolver. You can destroy the trigger or hammer but even with perfect aim, you're almost guaranteed to destroy the wielder's finger as well. You can blast a hole through the body of the weapon to destroy the internal gears, but you'd have to punch a hole through one or both of the wielder's hands before you could hit the body. You could aim for the cylinder itself but if the impact set off any of the not-currently-chambered cartridges in the cylinder, you could blow the entire weapon apart (along with the wielder's hands). The safest thing to target would be the barrel. Severe damage to the barrel would make the weapon unusable but wouldn't prevent it from being fired. If the wielder didn't realize the problem, attempting to fire the weapon could result in catastrophic damage that could maim or kill the wielder. All in all, there's just not a way to safely disable something with a projectile when it's covered so completely by the wielder's hands. The wielder is going to suffer permanent injuries, if not complete loss of a limb or of their life.

2.5mm accuracy sounds like a lot, but don't underestimate the size of your target. The firing pin for a Smith & Wesson 1911 9mm pistol (random example) has a radius of ~0.86mm. If you're trying to disable the weapon by breaking the firing pin, then the majority of your shots will miss your target and risk causing collateral damage instead. Triggers are a thin, flat strip. Line of sight to the broad side of the trigger will be blocked by the weapon body itself, so you'll be aiming at the thin profile of the trigger which is much smaller than 2.5mm. Aiming for similarly-small internal components is also risky, as there can be a fine line between disabling the weapon and causing it to discharge while pointed in an unsafe direction.

Not to mention, if loss of life or extremities is completely unacceptable, then that becomes a loophole that renders your entire system inert. Humans would simply re-design weapons so that all of the useful components are hidden behind the user's hand, arms, and body, or so that they will self-destruct violently if subject to extreme external forces. The drones will never have a clean enough shot and thus will never fire.

With these things in mind, your overall approach is doing things the hard way. Your drone should ignore the weapon and shoot the operator's hand directly. They'll end up with significant hand damage either way, but at least you'll avoid triggering a fatally catastrophic RUD event. If you really want to minimize operator injuries, have your drone disable the operator with a dart laced with some sort of incapacitating agent. 2.5mm accuracy is generally sufficient for targeting parts of the human body like the thigh or shoulder. You could also disorient them with a remotely delivered flashbang, immediately followed by several rubber bullets or beanbags aimed center mass to knock them down and disable them long enough to be disarmed. A drone-mounted directed energy weapon could heat up an attacker's hand and weapon until they reflexively dropped it, without permanent damage to the weapon or attacker.

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Let's use something... electro-magnetic with the aim to disarm.

Guns are mostly made of metal. It is hard to hold a metal thing, that is hot enough. If we manage to heat up the gun without cooking up the gun wielder, we are good.

So, instead of using a typical railgun to electro-magnetically accelerate a projective, let's induce Eddy currents in the target gun. Probably, magnetic field to do so is too high, however. It would be a device of a size of a MRT scanner.

Unfortunately, several Tesla of magnetic field strength might:

  • fry sensible devices, such as heart pacemaker;
  • yank small metal objects in the direction of the magnet: imagine keychains and paperclips flying with considerable speed;
  • yank the actual gun from the target,

which makes it not an ideal solution.

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Reverse Railgun.

A railgun uses electromagnetism to launch a metal projectile forwards. There is a big line of electromagnets that engage in sequence to follow the bullet as it moves along the barrel.

You want the railgun to shoot a rifle out of a guy's hands without hurting the guy. The problem is the bullet will go through the rifle and through the guy; or the bullet will stop and the rifle will keep going and go through the guy.

Of course that assumes we are using the railgun to launch the projectile forwards. . . .

I propose we instead launch the projectiles backwards. The magnets engage and instead of shooting a metal bullet, the railgun sucks the metal rifle from the guy's hands.

This disarms the rifle guy. More importantly it does not harm the rifle guy. Provided he lets go in time.

To Disarm and Not Harm. The motto of the United Railgun Corps for the past 100 years.

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    $\begingroup$ Railguns only impart force when the projectile is very near the electromagnet. An electromagnet that imparts a particular force at a distance of, say 0.1m, will need to be 400 million times bigger to impart the same force at 2km. Railguns aren't designed to suck. $\endgroup$ Mar 3, 2022 at 22:03
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    $\begingroup$ Magnetic fields fall off as the inverse cube of distance. Pulling a gun out of someone's hands at a range of even a few tens of meters requires an insanely powerful magnet; doing it at two kilometers is entirely unreasonable. $\endgroup$
    – Mark
    Mar 3, 2022 at 22:38
  • $\begingroup$ Please don't use this against someone with their finger on the trigger, otherwise rifle guy is never going to shoot that rifle or any other rifle in his life. $\endgroup$ Mar 3, 2022 at 23:28
  • $\begingroup$ @Mark "I am not interested in debating the feasibility or otherwise of a railgun & slugs with these capabilities." $\endgroup$
    – Daron
    Mar 4, 2022 at 0:18
  • $\begingroup$ @Blueriver Still better than the alternative! $\endgroup$
    – Daron
    Mar 4, 2022 at 16:47
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Railgun Big

enter image description here

The American Navy will have you believe their railgun can fire a 10kg tungsten projectile ten times the speed of sound, for hundreds of kilometres and when it gets there it will go straight through a tank and then through the tank behind the tank.

At least they USED to claim that. If I am correct they now claim to have stopped working on the railgun since the raily-ness was too expensive it didn't offer enough benefit compared to missiles and normal cannons.

The railgun was never good at hitting big things cost effectively. It is certainly not the sort of thing to pew pew a rifle out of a soldiers hands without hurting the soldier.

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    $\begingroup$ Uh... my question said that a railgun can be made as small as necessary to give a projectile of the necessary size the necessary kinetic energy. $\endgroup$
    – Monty Wild
    Mar 3, 2022 at 13:45
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    $\begingroup$ Based on the only real examples I don't think a small and long-range railgun is realistic at all. You can handwave the size issue of course. But then I don't know how to answer the question, without just handwaving the accuracy issue. Remember this is tagged as science based. $\endgroup$
    – Daron
    Mar 3, 2022 at 13:50
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    $\begingroup$ You can make a railgun at home to propel a wad of alfoil if you have the right gear. Anyway, the science required in this question is about bullet size, material and velocity to achieve a particular goal, not the feasibility of the launcher. $\endgroup$
    – Monty Wild
    Mar 3, 2022 at 13:55
  • $\begingroup$ @MontyWild Okay I will try the foil railgun in the morning. Only my garden is less than 2km long so I won't be able to check the range properly. Maybe I come over to your house tomorrow and try there? $\endgroup$
    – Daron
    Mar 3, 2022 at 14:53
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    $\begingroup$ Reasons for downvoting: (a) Didn't read the question. (b) Didn't answer the question. (c) Expressed skepticism about the American Armed forces and our beautiful boys in blue. (d) Referred to it as the "American" navy and not the US Navy. (e) Used metric units in an answer about American stuff. $\endgroup$
    – Daron
    Mar 3, 2022 at 14:56

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