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How would a non-lethal, hand-held ‘firearm’ (as in not a melee weapon) produce enough ‘force’ (I’m using this word lightly, not as a physicist) to throw an attacker back a minimum of ten feet, work?

Right now my understanding is that this would be impossible given our current knowledge of science / physics, but I’d like this firearm, in my fictional world, to not entirely be pulled from thin air.

This firearm does not need to be long-ranged (my original, non-informed concept was akin to a shotgun that shoots only highly compressed air strong enough to throw someone back without firing an actual projectile).

I have considered shockwave technology, like the “Thunder Cannon,” but none small / portable enough exist and even if they did, one strong enough to throw an adult human backward would also liquidity their insides and the recoil would likely kill the one shooting.

I’m asking for ideas, not breakthroughs: feel free to create any scenarios that allow your’s to work.

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    $\begingroup$ Your problem is that outside of sci-fi there is no such thing as a 'non-lethal' firearm. There is only 'less lethal' i.e. weapons that fire projectiles (usually short ranged) which if used correctly normally incapacitate the victim without causing death or serious injury. (Emphasis on 'used properly' and 'normally'.) For example there are many kinds of non-lethal rounds which can potentially cause fatal injuries if the victim is struck in the head instead of the body as is recommended by the manufacturers. Same goes for elector-shock or infra-sound weapons etc 'normally' they won't kill you. $\endgroup$
    – Mon
    Mar 18 at 12:53
  • $\begingroup$ hehe well in our world that is indeed so, @Mon.. but this question has an SF tag. I wonder if you would make e.g. the bullet diameter VERY big ... with a spherical near-flat top, what happens? I'm definitely no expert on the subject, but if the victim has a bullet proof vest, a very thick vest, or thick clothing.. the force may be considerable. Also SF technology could enable to dose the power applied to the bullet very accurately. With Earth firearms, you don't have that option. $\endgroup$
    – Goodies
    Mar 18 at 13:09
  • $\begingroup$ How sci-fi do you want to go? A tiny drone with a claymore-like charge could fly up to someone, apply a flat surface to them, then explode like a rocket. The likelihood of harm is high, but it wouldn't even NEED to be a fiery explosion (those are just easier). A mini-air-fuel bomb might cause a concussion big enough, but would probably do bad things to organs. $\endgroup$
    – DWKraus
    Mar 18 at 14:00
  • $\begingroup$ Your understanding is correct. This needs the magic tag if you want an answer that isn’t “no.” Throw a 65kg bag of sand at yourself from 10 feet away. Now, hold out your hand and stop it. This is exactly what the gun will feel like when you fire it. Without magic, of course. $\endgroup$
    – Vogon Poet
    Mar 18 at 18:00
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    $\begingroup$ "A couple of feet" is actually possible with real-world physics. You would just have to adapt the principles of a Recoilless Rifle. I agree that "10 feet" is likely to be somewhat lethal. $\endgroup$ Mar 19 at 14:47

17 Answers 17

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The real world problem you will run into here is conservation of momentum - if the projectile has enough momentum to knock the target back ten feet, the recoil of firing it has enough momentum to knock the firer back ten feet.

Then if we consider falling ten feet as an approximation of the required impact, non-lethality looks questionable...

However, can we deliver enough momentum to at least knock some one over without risk of serious injury?

Perhaps...

It wouldn't look much like a firearm - more like a rocket launcher. Rockets don't recoil because the propellant only pushes against the rocket, not a bullet AND the breech face of the weapon firing it, with equal and opposite force.

Now we can fire a large heavy projectile with sufficient momentum to have a chance of knocking the target back or down. How do we stop it killing them? First launch it slowly. Second, fit an airbag triggered by a proximity sensor. Given our large projectile there should be more than enough room inside.

The target will effectively get hit by a heavy pillow moving at baseball pitch speed, which could do the trick. Such a thing couldn't exactly be considered safe - getting hit in the head would probably snap the targets neck like a twig, but hits to the torso or legs are unlikely to do serious harm. Just watch out for the back blast behind the firer - it will be considerably more dangerous than the projectile!

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    $\begingroup$ Or give it two stages. The first stage propels it relatively slowly. The second stage ignites when it hits the target. $\endgroup$ Mar 18 at 21:03
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    $\begingroup$ The same argument applies to a punch. If it knocks the target back then it should knock the thrower back too. It doesn't, and it's in the way the force is transmitted and absorbed. $\endgroup$ Mar 18 at 22:26
  • $\begingroup$ One think I wonder about is the chance of crushing your skull on the ground, when you are smitten back several metres by a projectile cushion. Falling backwards is risky even when you're standing still. $\endgroup$
    – Cerberus
    Mar 19 at 2:45
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    $\begingroup$ @DJClayworth: A punch does knock the thrower back. It's just that we tend to punch while moving forward so we're decelerating from forwards, instead of accelerating to backwards. You could fire the gun while moving forward to get the same effect, except that you're basically sprinting so the counter-force just stops you. And it would still hurt a lot because the recoil is twisting from points on your body instead of being spread across your body. $\endgroup$
    – MichaelS
    Mar 19 at 9:27
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    $\begingroup$ Also note that when a punch "knocks someone back" it's because we knocked them off balance and they step back or topple over. You'd be very hard pressed to punch someone your mass hard enough to literally throw them backwards more than a few inches if the center of mass was low to the ground. $\endgroup$
    – MichaelS
    Mar 19 at 9:31
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Is it really non-lethal?

If your weapon can knock the target back a few feet, there is a good chance they will hit their head, and thus it cannot be a truly non-lethal weapon. "Non-lethal" is really a misnomer, one sometimes used to validate police attacking civilians, but anyway...

Use the elements

Water cannons/firehoses have been used for riot control.

Maybe some kind of souped up airzooka would work too Especially if the target was wearing baggier clothing that could work like a sail.

enter image description here

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    $\begingroup$ Non-lethal is not so much a misnomer as it is simply misunderstood. Getting knocked over will probably harm you (expect possible whiplash, concussion, and or broken bones) but is very unlikely to actually kill you. Non-lethal does not mean non-harmful. $\endgroup$
    – Nosajimiki
    Mar 18 at 15:57
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    $\begingroup$ Tasers are considered as non-lethal. Non-lethal means "this will usually not kill you unless used too much or improperly", not "this will 100% make you gently ragdoll into a soothed state of nonagression completely unharmed " $\endgroup$ Mar 18 at 16:33
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    $\begingroup$ As the OP has revised the throw distance to 10 feet, we're now talking "hit by a truck" kind of impacts. This is entering Batman levels of "non lethal". $\endgroup$
    – jdunlop
    Mar 18 at 18:28
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    $\begingroup$ A canon that shoots trucks $\endgroup$
    – Atog
    Mar 18 at 19:19
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    $\begingroup$ @jdunlop Remember that sonic shotgun from Minority Report? That thing was sending people flying like 4 feet up and 8 feet back lol. $\endgroup$
    – DKNguyen
    Mar 18 at 19:56
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The Turbo-bolo-net

Lots of solutions are being posted but everyone has to accept the basic laws of momentum. You have 65-ish kilograms being thrown 10 feet, Isaac Newton demands that an equivalent mass be thrown in the opposite direction. This can either be a heavier mass thrown with less power, or a lesser mass with more power. In the end, the (mass times acceleration) on the left will be the same as the (mass times acceleration) on the right. We can’t escape this.

So, you’re firearm tangles the bad guy in a net. On the corners of the net, drone pods light off and drag the net with bad guy to the wall. These little drone pods are little turbo fans on steroids. Likely they burn up with a single use, but they create such a powerful thrust of air that they could literally lift the bad guy up several feet before shutting down.

The fans should be as simple as possible, and use no fuel. Let’s make them run by a steel spring wound to several thousand PSI, and geared to the fan blades by amazingly tough gears, in a hardened titanium casing. Now nothing is burning, it’s just air.

The net and its four or six turbo fans fit into the end of your arm weapon, and are catapulted toward the bad guy. Fans fire up, net spreads out, you’re out of danger.

So where is the 65-ish kilograms going backwards? It’s very light, but very forceful air. Very possibly dangerous thrust if your hand gets in the way, but I can’t be sure. Definitely bad if the intake hits anything.

There is a lot of complexity in making this thing go in the direction you want, but at least Isaac is happy. Some other engineer will have to make it go straight.

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  • $\begingroup$ Wow, this is almost exactly my answer, and I can't tell who put theirs in first. Capture and launch is the only solution I can think of with that kind of range. $\endgroup$
    – DWKraus
    Mar 18 at 23:47
  • $\begingroup$ @DWKraus Ah I didn’t read to the bottom of yours because it involved drones and rockets, but yours was first. I caught the first few paragraphs with wrapping and tressing up (a la Spiderman) and decided I was on a different page. Sorry. I’m using the word “drone” very liberally here, no AI or anything. Just enough to fly as straight as possible wit the net. $\endgroup$
    – Vogon Poet
    Mar 19 at 1:09
  • $\begingroup$ "Newton demands that an equivalent mass be thrown in the opposite direction." <- This is an oversimplification. How you brace against a force makes a huge difference. $\endgroup$
    – Nosajimiki
    Mar 20 at 1:22
  • $\begingroup$ @Not really? Even a perfectly rigid catapult will throw the boulder one way, and spin the earth the opposite way... a teeny weeny bit. Equivalent M*a will happen. $\endgroup$
    – Vogon Poet
    Mar 20 at 2:19
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You need to distribute force over time

The difference between a punch and a shove is that a shove distributes the impact over time. With bullets, no matter what you make it out of, this is very limited. Even rubber bullets or bean bags would cause major trauma long before they shove a person back because you only really have a few millimeters of compression before the projectile become ridged enough to pernitrate the target.

Instead you need to transfer the force using a fluid. Water works very well as you see with a Fireman's hose, but they are not portable; so, what you really need is a very powerful water gun. The strongest commercially sold water guns in the world use CO2 cartages like you use in a paintball gun.

A typical paintball CO2 tank weighs about 24grams with a total potential energy of about 432J. Because a CO2 tanks loses efficiency as it decompresses and needs time to warm back up, the most sustained energy you can get out of a CO2 is about 1/4 of its total capacity over 5 seconds. IE: 20 J/sec.

It generally takes about 80 J/sec. to even start pushing a normal grown person backwards. So, lets say you want to push someone back 10 ft over the course of 10 seconds, using a CO2 powered water gun, you would need to increase your total potential energy up to about 7000J which would need to be stored in a series of CO2 tanks that are opened in sequence to levelize out to a sustained push as the CO2 tanks lose pressure and cool down. Water guns also suffer a lot of energy loss over distance; so, while a 7000J tank system may work at near point-blank ranges, you may need several times this for a longer ranged water cannon.

First, you will want a more efficient CO2 tank. A portable sized 3000 PSI fiberglass CO2 tank translates to about 80 Joules of potential energy per gram of weight after you account for inefficiencies like the weight of the tank itself and escaped gases, meaning you store the force required to push a person back 10ft from point-blank range with a set of CO2 tanks that weigh just 6lb. But, water guns are not super efficient over range; so, if you want to knock someone back from farther away like 10-20m, or someone who might weigh a bit more than average, I would expect you to need something more like a 20lbs of CO2 tanks at least.

The harder part would be the weight of the water. A high end CO2 water gun can fire 30oz of water per second (usually in burst of less than 1 second). The higher pressure CO2 tanks means you could achieve much more force per mass of water; so, to get 8 times the force of a normal CO2 water gun with 4 times the pressure, you need 60oz of water per second. So a 10 second burst would require 600oz of water (37.5lb).

So to give yourself a 10 second burst "portable fire hose" you would need about a 20lb CO2 tank, a 40lb water tank, and probably 5-10lb for the actual water gun, tubes, harness, etc. giving you a total weight of just around 65-70lb.

While this sounds unreasonably heavy, keep in mind that the M1 Flamethrower weighed about 70lb and could only fire for about 15 seconds; so, basically what you are making here is a non-lethal flamethrower. Also, the weight of the weapon system will help offset the significant recoil. Can't say if this would be practical enough for your setting, but it would get the job done.

It's also worth noting that getting hit with over half a gallon of high velocity water a second is still going to hurt quite a bit. Your target will likely sustain bruising anywhere the water hits him, and possibly break a few bones as he is knocked back and tumbles from the force of the spray... but it would be very unlikely to kill a healthy grown person; so, should still meet the qualifier of a "non-lethal" weapon.

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  • $\begingroup$ This could also maybe be used as a good counterweapon to a flamethrower $\endgroup$
    – Atog
    Mar 18 at 19:23
  • $\begingroup$ The pressure (force/area) is also important, since the entire body can handle more force than any single part of it. Something along the lines of a net with elastic properties could do this. $\endgroup$ Mar 22 at 2:08
  • $\begingroup$ @MaxCandocia The water does not need to be in a perfect beam. It will naturally want to spread out a bit. If you design your nozzle with this in mind, you can make sure it spreads out enough to minimize injuries. $\endgroup$
    – Nosajimiki
    Mar 22 at 14:03
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Boxing glove on extendable linkage

enter image description here

Imagine a steampunk or cyberpunk version of this cartoon classic, but powered by steam or pneumatic pistons, and using advanced alloys that can give it a range of 5m.

Not technically a ranged weapon, but

Boxing glove rocket

enter image description here

As a last ditch effort, you could launch the boxing glove. Just be sure to bring extra ammo!

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  • $\begingroup$ This is almost the right answer, except that instead of a boxing glove, the device needs to shoot out "a couple of feet". Each one of those being about five feet long, I suppose, to get the right overall distance. Soft, colorful, well-shined, insistent, a little humiliating, and perfect for kicking people out of places where they haven't been invited without having to make a federal case out of it. $\endgroup$ Mar 19 at 17:28
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I think it is possible, with a squishy enough bullet: imagine something between the size of a volleyball and a basketball, made with a gelatinous substance which splashes around the target once it hits it.

On impact the contact force will spread on a large enough area to be fairly non-lethal if properly aimed (aside from maybe broken ribs or dislocated jaws), and the imparted momentum will be enough to kick the target back or send them to the ground.

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  • $\begingroup$ +1 agree bullet diameter would do the trick, mainly. I wonder what is required to let it work with a fire arm.. the weapon will need a special design ! Square cube law also counts for explosions. The bigger the bullet, the bigger the bang (ignition, explosion) needs to be. Maybe a golf ball would work ? As long as it not penetrates.. $\endgroup$
    – Goodies
    Mar 18 at 13:17
  • $\begingroup$ Yes a bit like a rubber bullet $\endgroup$
    – Slarty
    Mar 18 at 19:12
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The Humble Gyrojet Balloon Cannon

enter image description here

Standard guns put all the energy in the bullet when it is in the gun. That means the shooter gets the same total force as the target.

The gyrojet instead fires a small rocket that accelerates AFTER leaving the gun. This means the gyrojet has less recoil at the cost of added complexity.

So rather than using a big compressed air cylinder to fire a big rubber balloon, use a small cylinder to lob a balloon that itself contains a smaller single-use cylinder. This cylinder fires midair and launches the balloon faster at the target without launching the shooter backwards with the same momentum.

This is still a lethal weapon since the target might hit the back of their head and die and it's all your fault. No matter how good you are with the gun, you have no control over how and where the person you are attacking will land.

The issue is how hard how hard do I have to hit them before they run away. It might be safer to launch a smaller balloon that breaks an arm or leg without propelling the target across the room.

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  • $\begingroup$ This is a good idea, however a ballpark estimate of how much compressed air we need to achieve the desired result indicates that our weapon would likely be a single shot one. $\endgroup$
    – Alexander
    Mar 18 at 16:38
  • $\begingroup$ @Alexander I imagine the ammunition is stored outside the weapon and after firing you take another one off your belt and click it onto the front of the barrel. $\endgroup$
    – Daron
    Mar 18 at 18:13
  • $\begingroup$ Actually gyrojct pistols were tried out, during the Vietnam War, I think, and were discontinued because their rounds were non lethal except at very close range. Possibly a version could be designed to be less lethal but more stunning. $\endgroup$ Mar 18 at 19:55
  • $\begingroup$ @M.A.Golding I'll well believe it. Gyrojets are a stupid idea for replacing conventional firearms. $\endgroup$
    – Daron
    Mar 19 at 13:07
  • $\begingroup$ @M.A.Golding The reverse, actually. The rounds were just fine (other than being overly expensive, somewhat failure prone, etc) once they had enough time to accelerate, but if the target was too close they wouldn't have built up enough momentum and wouldn't do enough harm to the target. $\endgroup$ Mar 21 at 18:26
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Rocket Propelled Net

If you have a net tucked into a rocket capsule, I think it is feasible to fire such a system from a normal looking rocket launcher. Presumably you could manually set the net to deploy at a desired distance as well. The net would be able to distribute the force over a very wide area, allowing you to hit the target much harder than you could with a smaller projectile. With somewhat elastic net material, you could even distribute the force over time (like a trampoline), meaning you could push them a lot harder.

However, while this could theoretically be non lethal if properly deployed, anything that is able to knock someone off their feet could very easily kill them on accident. If the net didn't deploy, if any solid piece of the rocket didn't get out of the way in time, if the net picked up something solid and rammed it into the target, or if the target falls over and bashes their head, you will have to find a body bag.

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Police Drone Capture Bots:

Your average future police force is dealing with some pretty savvy rioters and criminals, Not to mention that police officers are outnumbered and in constant danger of being sued for excessive force. So to solve this problem, they've built capture bots to grab a hold of criminals and resistant citizens. If they resist too much, there's a rocket-assisted thruster to clear them from danger.

These bots would be relatively small, but have flexible cables or sheets to enclose the limbs and heads of humans, leaving average citizens trussed up and ready for capture. They are smart enough to analyze behavior and adapt to keep their charges safe. If they need to remove a person from a situation (like thrusting them back from a police line or moving them out of the way of an oncoming car) they can't pick them up (they're too small) but they do have a semi-rocket/explosive charge to generate a LOT of thrust. So after securing a human's neck and analyzing the risk of injury to the human, they can blast the human out of harm's way with great force.

In the event of a serious injury to the human, they are equipped with life-saving diaphragm and heart-stimulating equipment. They then call the paramedics for emergency assistance.

But accidents happen, as nothing is perfect. It was the effort to save them that killed them, not the actions of the police. ;-)

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This sounds similar to an existing less lethal weapon called a bean bag gun (in simplest form, just a special kind of ammunition for a shotgun) that is used by law enforcement for riot suppression and in situations where someone is a serious, but non-lethal threat to themselves or others.

You can watch a YouTube vide of one in action at this link.

You might want to tweak it with more power, but a more compressible or larger diameter projectile, to get something close to what you are looking for.

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I have heard accounts of at least one type of weapon that might non lethally force a human backwards a distance of 10 feet.

The type I am thinking of would probably not be usable by human beings, and it would probably not be usable in most story situations. But possibly you might be able to use it in a story sometime.

As well all know, toothed whales, dolphins, etc. use sound for echolocation. They use it to locate prey amoung other uses. Sperm whales are the largest, stongest, and most dangerous predatory animals alive today. And to gild the lily, there are also theories, with greater or less degrees of scientific acceptances, that sperm whales use their sonar to stun prey, or even to kill it, or even to predigest it!

You may also know that researchers have dived with sperm whales, despite the sperm whales being by far the largest predatory animals living at the present time. And I have read somewhere on the internet that a diver was pushed backwards several feet in the water by the sound pulses from a sperm whale. I have also read that some divers hae felt their bodies starting to heat up as they heard and felt sonic pulses from sperm whales.

Of course sound travels much better and is much louder in water than in air, so even it was was practical for someone to carry a 50-foot sperm whale in their holster, the whales sonar would have much less effect in the air.

Of course a much smaller animal than a sperm whale could use their snout to push someone backwards a dozen feet in the water, and if they pushed hard and fast enough they might temporarily stun the person, who might or might not drown.

One time I was talking a walk and a collie came behind me and pushed me forward, apparently trying to get me away from their property as fast as possible. I have also known a huge billy goat who cold have pushed someone a distance, and maybe stunned them while doing so. Of course there are land animals much larger and stronger than collies and billy goats.

You migth also want to consider the rocket pictols tried during the Vietnam War I think. They were considered ineffective because they were nonlethal at anything but the shortest ranges, I think.

I have also seen a design for nonlethal bullets in one or more anime. The shape of bullets is designed to fit into gun barrels, and also to penitrate bodies easier and do more damage.

These ficitonal bullets had 4 long arms which were folded back parallel to the bullet and the shell when the round was loaded. After leaving the muzzel they would more out perpendicular to the axis of the bullet, due to springs or the spinning of the bullet, and would probably lock into place. Thus when the bullet hit the target, the four arms forming a cross shape would increase the surface area of the bullet and prevent it from penetrating into the body. And for all I know there could be a theoretical or real such design that inspired that anime.

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Like an airsoft gun but with balls filed with very high-density air. On impact it should burst, releasing a wave of air in all directions potent enough to produce the said punch. The technology needs to make sure balls actually burst, or else you will get a regular (rubber bullet) gun.

Not sure how many atmospheres do you need to have inside to produce this effect.

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  • $\begingroup$ That still means that all the force is delivered on the impact point. You'd need the ball to burst before impact. $\endgroup$
    – jdunlop
    Mar 18 at 18:30
  • $\begingroup$ No, it is not delivered on the impact point, it is delivered across a wide surface. But maybe you would actually need to burst it before the impact for best bang. $\endgroup$
    – alamar
    Mar 18 at 18:36
  • $\begingroup$ What you're describing is an explosion: "a violent expansion in which energy is transmitted outward as a shock wave." Doesn't matter if it's an explosive or the expansion of something under pressure, if it bursts on impact, it's an explosive bullet. Compare setting off a stick of dynamite at ten feet from a wall to setting off the dynamite in contact with the wall. It's not delivered across a wide surface if it explodes on impact. $\endgroup$
    – jdunlop
    Mar 18 at 18:39
  • $\begingroup$ Basically an explosion, but without fragments or flame. $\endgroup$
    – alamar
    Mar 18 at 18:41
  • $\begingroup$ An explosion needs neither. I included the definition. The shock wave alone would be enough to cause internal damage at that range, concentrated on that small a part of the body. $\endgroup$
    – jdunlop
    Mar 18 at 20:24
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Automate existing police tactics

I will answer the title question and describe a method currently in use, which could move the target back a few feet, but maybe not exactly ten feet as asked in the body of the question.

Instead of using a brute and dangerous physics solution involving reaction mass, leverage the tools you already have. There is a suspect standing on their own legs, and possibly walking or running.

When a law enforcement needs to gain compliance from a suspect, they have two techniques which may, with very complex technology, be combined to literally make the target jump back several feet using their own legs.

Step 1: Impact the Common Peroneal Nerve (side of leg)

When a suspect refuses to put their hands behind their back for cuffing, an officer is trained to thrust their knee into the side of your thigh, between the hip and the knee. The nerve in your leg will be disrupted, and your knee will fold under you.

So your firearm needs to be able to apply some rubber ball or soft device to this precise location on both thighs. If you gun had very advanced computer imaging and targeting, it may be able to calculate exactly how to launch a bolo-like pair of weights so they strike this nerve precisely. It’s very likely the gun operator will need specialized training and practice with this weapon to be accurate, and they need to know how to force the target into a good position for a strike.

Step 2: Taser to the legs

Just as your target is collapsing on their knees, your gun shocks their thighs with a taser and forces the quadriceps to contract completely. This will extend the legs with all the might their muscles can deliver. Since the target was already falling on their knees, the result will be the target jumps back like a grasshopper when their legs spring out from the shock.

Now the force to throw your target back is delivered by their own legs, and the gun is easily manageable.

This weapon takes a lot of training to ensure it is not dangerous. The target should not be moving forward quickly or they will lunge toward you instead of backwards (this isn’t entirely bad, they will still be on the floor). So have your operator well trained before using this weapon.

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Is it strictly necessary that the weapon throw the target back 10 feet? Or is it sufficient to say that after firing the weapon, the target ends up 10 feet farther away?

You should be able to accomplish the latter with a directed energy weapon. Such weapons send a beam of microwave radiation at the target, heating up the outermost layer of skin until the target retreats from the pain (often doing so reflexively). Once out of the weapon's range, the pain instantly ends and there is no long-term damage. A rangefinder on the weapon can measure the distance to the target and only deactivate after they've retreated the desired distance. Current models are large vehicle-mounted units, but they're designed to work against large crowds. A lower-power version designed for 1-on-1 use could conceivably be miniaturized to something that's man-portable.

The end result of firing such a weapon is that the target is 10 feet farther away than where they started. They moved those 10 feet under their own power, but your weapon forced them to do so. You're simply using thermal energy instead of kinetic energy.

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How about a gun that shoots projectiles that baloon up in size to the size of a basketball a short distance before hitting the target. Such projectile would distribute the force much more evenly and, bonus, bounce around too onto anything that's nearby. It could be a plot device to shoot around corners and you could even have automatic rifles of these things. Maybe you could even have swords used as defensive weapons to "pop" such projectiles. Friendly fire would also be particularly dangerous as the projectiles wouldn't have enough time to fully expand, allowing for more story telling.

The recoil shouldn't be too different from that of a normal gun.

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It could be an expanding projectile which is a folded frame similar to a Hoberman sphere. However, instead of going from a small sphere to a large sphere, it would be more suitable for launch from a gun/cannon if the projectile went from a rod shape (the size of a baseball bat) in the folded configuration to spherical shape (the size of a gym ball) in the unfolded configuration.

Explosive charges at each joint rapidly expand the projectile. Because the frame is made of a flexible material, the momentum from the projectile is transferred relatively slowly from the projectile to the person which could enhance the push (similar to a person getting hit by a gym ball).

A recoilless rifle could be used if recoil was a concern.

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The momentum of the projectile has to be large but this doesn't necessarily mean that the recoil force needs to be large (although this is often the case). If you accelerate slow enough the maximum force won't be too large. Maybe some sort of rail gun to have high control over the acceleration of the projectile. The barrel would probably be quite long, to increase the accerlation time, and thick, to accommodate the projectiles. The projectiles would probably be large and massive but quite soft to not kill the reciever on impact. Or maybe the projectile could be a net with bolas.

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