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In my world personal body armor has reached a point where most weapons designers have given up trying to make cartridges that will penetrate armor, and instead have gone back to large caliber rifles that are optimized for transferring energy through the armor to the soft squishy tissue of its wearer. Most military rifles, in this world, have between an 8-12mm bore diameter and the weight of most projectiles is around 250-300 grains. With standard propellants, equivalent to what we use today, the velocities of these projectiles can be expected to be around 700-800m/s, increasing muzzle velocity would require a larger cartridge and these rounds are already getting pretty large for rank and file infantry to haul around as is.

So with that background in mind this is my question: Is blunt energy transfer, instead of creating wound channels, with large projectiles a reasonable method of killing your enemy; and how effective are we at reducing energy transfer through body armor with near future technology?

In universe considerations: The factions in question for this period have just begun colonizing other planets. Most infantry-infantry combat takes place planet-side on frontier worlds, ammunition often cannot be produced in the field and must be packed in, thus, individual marksmanship is valued over high volume of fire. Since most combat will take place on these frontier planets soldiers may have to contend with large and dangerous wildlife in addition to enemy soldiers, thus another, although less important, reason for larger rounds with good stopping power. This is a soft sci-fi/ science fantasy setting, some handwavium is acceptable.

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    $\begingroup$ Similar to my question here worldbuilding.stackexchange.com/questions/228781/… I didn't get a good answer on how hard it is to turn armored soldiers into a milkshake. I do feel like mass of armor is key $\endgroup$
    – Andrey
    May 13 at 17:21
  • $\begingroup$ How effectively this armor spreads out the momentum? Is it by its own weight alone? What about headshots? $\endgroup$
    – Alexander
    May 13 at 17:38
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    $\begingroup$ Is it too handwavy to have the bullets increase in mass during flight? $\endgroup$
    – Mathaddict
    May 13 at 19:43
  • $\begingroup$ @Mathaddict Lol yes. I do love the idea of the bullet getting exponentially more massive the longer it stays in flight though. Imagine shooting at a ship off the coast and by the time it reaches it it's the size of a large torpedo. $\endgroup$
    – Boo Radley
    May 13 at 20:08
  • $\begingroup$ So there's no mechanised exoskeletons, right? Because otherwise you could carry around cannons. $\endgroup$ May 16 at 2:02

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Armor is designed to spread out an impact over a larger area. This is how if you step on a nail it will go through you foot but if you can lie on a bed of nails without injury. If armor is somehow so good that penetrating the armor is impossible generating enough force to injure someone through armor will become incredibly difficult.

You can definitely be killed by blunt force applied over a large area, that's what happens when a car crashes into a pedestrian, but, the amount of energy involved in that lethal crash is far far higher than the energy needed to put a hole in someone with a bullet. Remember that firearms generate an equal opposite force to that imparted into the projectile. Recoil sucks but it isn't enough to kill someone since it's spread out over a larger area. If you need an energetic enough projectile to crush someone to death the recoil will also need to be enough to crush whoever holds the weapon as well.

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  • $\begingroup$ So it's sounding like I'll need to include something in that big round as a payload to release more energy on impact without obliterating the shooters shoulder. I cant imagine shaped charges are going to be the way to go on small arms so maybe explosive rounds could do the trick? Or you could go all the way and have the whole squad built around working with a heavier crew served weapon like a grenade launcher or lightweight tripod mounted cannon. Would this be an effective way of getting around really good armor? $\endgroup$
    – Boo Radley
    May 13 at 17:34
  • $\begingroup$ "Firearms generate an equal opposite force to that imparted into the projectile": Yes they do, but because that dastardly Sir Isaac Newton and his accursed conservation of momentum the vast majority of the energy goes into the bullet (to be transferred to the target) and only a small fraction goes into the firearm (to be absorbed by the shooter). $\endgroup$
    – AlexP
    May 13 at 17:34
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    $\begingroup$ @AlexP I don't think that's right. The momentum/energy is equal between the weapon and the bullet. The weapon is just much much more massive, so the bullet travels faster. The heavier the gun, the less recoil on the user. So once the armor which let's say is as heavy as the gun dissipates the impact, the damage should be equal/less $\endgroup$
    – Andrey
    May 13 at 17:43
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    $\begingroup$ @Andrey Momentum is equal, but energy is not, since kinetic energy is proportional to velocity squared. A bullet with 1/100th the mass of the gun is fired at 100x the speed of the recoiling gun and has equal and opposite momentum to the gun, but has 100x as much kinetic energy as the gun does. Even though the momentum change of the shooter and the person being shot are identical, all that extra energy has to be dissipated somewhere in the target, either by deforming the armor, or sending a damaging shockwave through the body. $\endgroup$ May 13 at 17:52
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    $\begingroup$ @NuclearHoagie But if a projectile is energetic to send impact the armor with enough force to send a fatal shockwave through the armor without penetrating it, then the recoil should be energetic enough to send a fatal shockwave as well. $\endgroup$
    – sphennings
    May 13 at 17:58
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The answer depends on why armor is so good.

If armor is just freaking thick hard pieces of stuff, then whacking it hard makes sense. That energy goes somewhere.

Tanks got past extra hard whacks with reactive armor - the armor pushes back against incoming force with a whack of its own. This is a plausible scenario for body armor. Or armor that electromagnetically decelerates the projectile - again a push back.

Another plausible scenario is an energy transfer system whereby kinetic energy is redistributed through the entirety of the armor - a super sand bag. Existing systems do this with glass beads. You would have to be hit so many times the armor got too hot to wear. Cooling fins would be a must, and big ones.

A more scifi scenario could be armor that converts kinetic energy into chemical energy - perhaps a phase change or reversible reaction. If you could let the chemical convert back on the fly you could have hits from your enemy charge your onboard battery. Harder hits power up your opponent more. Don't want that.

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The weapons you are thinking to kill a man would need so much energy that they would have a very strong recoil, they would be too difficult to handle and won't be very effective. Probably people would try to develop different weapons.

The easiest alternative would be self propelled bullets, in this way not all the push would go in the recoil, but with them unless you aim to the head one single hit would not be enough to kill.

Something more advance could be small anti tank missiles delivering the blunt energy transfer from a direct explosion.

The strange alternative could be a bullet that sticks, glued or with a magnet, to the target and then burns forcing the wearer to remove the painfully hot armour.

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Incapacitate

Consider that death is just one way to achieve your goal. Incapacitating your enemy is often the overarching goal! Though I'm sure that you can kill via blunt force trauma with bullets against future body armour, incapicitation is much closer. It is sometimes more effective too.

A body armour isn't as in the movies where they shrug off small arms fire. A hit with a run of the mill pistol can already leave a bruise bigger than an outstretched hand. Some future armour will probably protect them better, but your soldiers will have assault rifles and bigger. There will definitely be blunt force trauma. As with any blunt force trauma, many hits can take you down no problem. Boxers work on this principle, where damage to the body and preferably head will incapacitate the opponent over time.

On the battlefield many will still be able to continue to fight after a hit. They are filled with adrenaline, increasing pain thresholds and their will to continue. But now imagine a soldier hit, leaving a 20cm bruise. He might still be able to fight later, but it takes a big toll. If the soldier is hit multiple times or just badly the first time the person is likely to be useless after the adrenaline leaves the body. They need medical attention, draining resources and time. Depending on the amount and where they are hit, this can be a long time. Imagine this at a front with thousands of soldiers and it can become a logistical disaster. Technically you're better off with many dying instead of tying up huge amounts of resources for big hospitals and transport for medicine, extra food and much more. If not done correctly, this can take an army off the map thanks to the logistics problems. Dead people need to be burried or transported back, injured need to be properly taken care of.

Though the difficulty to kill directly is heightened,it is still a possibility. A few good hits and they are incapacitated, dying or dead. All are desirable outcomes in war.

Reducing the energy

Reducing the energy of a bullet is difficult. Newton's law tells us that every action has an equal opposite reaction. Simplified this means that the one firing the bullet experiences the exact same amount of energy as the ine being hit. The reason they are much better off is that the arms can move more easily to mitigate the impact, as well as the acceleration is just a bit longer than the deceleration against the body.

To reduce the impact you need to reduce the speed over a longer time, against things that can move more easily to take the impact. As it's hard to control where the impact will occur, we can mostly just add more layers. We can also try to spread the impact even further, but this might be difficult.

Because of this it is best to employ 2 tactics, which are often employed. First is not getting hit. That is why tactical advantages are so important. You want to maximise the ability to hit your opponent while minimising your own.

If you do get hit, deflect the bullet. Much like a helmet. You don't want to stop the bullet, you want it to impart as little energy as possible! Helmets, tanks and a lot more are all designed to do so. Thanks to oblique angles you either push the bullet on a different path, receiving little energy yourself, or the bullet has to plow through much more material as it's set at an oblique angle. Either way you have some advantage.

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As sphennings says, recoil is a killer. Literally in this case. Thus you must take action to reduce that recoil. There are two basic ways to accomplish this: muzzle brakes and sending the energy behind the shooter.

Muzzle brakes divert some of the propellant gas sideways and somewhat back--Wikipedia says they top out at about 50% of the energy, that's nowhere near enough for this scenario so let's forget them.

That leaves only expelling it over the shoulder. This is how man portable rockets work--the launcher is open in back and the rocket exhaust goes out behind the guy who fired it. It's quite dangerous to those nearby and quite dangerous to the shooter if there's anything behind him to reflect it (Note the video out of Ukraine--we see people with Javelins on rooftops, but never inside the buildings.) However, there are also rounds which trade a weight penalty for avoiding the reflection problem--instead of rocket flame blasting out the back they expel a counterweight. The counterweight is designed to turn into tiny pieces when used so you don't basically have a rear-firing cannonball. (A simple example of something that works: water.)

Thus you don't have a rifle. You have a barrel that goes over your shoulder that a round is loaded to a central point. The round consists of a bullet, propellant and the counterweight. There is no bolt behind it, both the bullet and counterweight are thrown with equal force, but the counterweight disintegrates upon firing. The process to load the next round will be far more complex than a modern rifle because you need to open the side of the barrel--that has to be a very, very strong closure!

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Explosive rounds.

The shockwave of an explosion is kind of a blunt force, the shrapnel is sharp, but the armor will likely deal with it. But the shockwave itself, oh no, that armor won't stop it. No armor will ever be perfect. It will have gaps somewhere, thin points, joints, general weaknesses, and thats where that explosion is gonna shine. For the shooter, not for the one getting shot. The projectiles would need to be quite large to contain the number of explosives needed, but hey, youre already firing rounds that are near .50bmg rounds (12.7mm, ~1000m/s), and they can use explosive rounds (the most interesting of which being the Raufoss Mk 211, look it up its wild). theres no reason that i can think of that these ones cant be HE, besides that its a war crime, but hey, times change.

The issue with using just large kinetic rounds with no extra source of energy, like from an explosion, is equal and opposite reaction. To send a huge projectile out of the barrel, you need a lot of force, and the same amount of force is gonna push back (assuming we are using conventional rifles). the gun can be engineered to help mitigate some of that energy, but it has to go somewhere. At a certain point, you wont be able to fire it without breaking your arm.

I don't care what futuristic armor you're wearing, taking an HE round to the chest is gonna knock you off your feet, and you'll probably stay down.

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