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It should not resist 50 cal bullets, but an armor which could protect a creature from the US standard issue rifle, for example. Here are a couple of things the armor needs to do:

  1. Protect the creature from the bullets mentioned above. Take a look at @computercarguy's answer for more information on the bullets I am talking about.
  2. The armor could be created from a chemical in the air into a solid piece of armor, i.e. the atmosphere contains some needed component for the armor. This chemical does not need to be in the Earthen atmosphere, it just needs to be some sort of gas, I'll work out the details of how it got into the creature's atmosphere. Note this is more of set dressing than anything else.
  3. The armor should not be much heavier per cubic inch than steel. The creatures could take more, but they still need to be able to lift quite a bit around.
  4. The armor could theoretically be "fixed" within a month at the max, but a little longer would be fine. Not a hard requirement.

UPDATE: I am looking for two specific things the other answer is not looking for, repair time and a request for it to be based on things in the environment, a theoretical element which helps build the armor.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Nov 3 '17 at 1:51
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    $\begingroup$ There's a storytelling issue with your question: you want to use military equipment, but military is an adaptable force with many tools at its disposal. When they realize that 5.56x45 with 1700J muzzle energy is useless what stops them from switching to 7.62x51 with 3800J or all the way to .50 cal? What about AP rounds with hardended steel or even tungsten cores? It would be more easier if you'd settle for pistol rounds(300-500J) and civilian protagonists and kept military in reserve as a cavalry/ex machina. $\endgroup$ – Nick Dzink Nov 4 '17 at 23:26
  • $\begingroup$ @NickDzink True, but some of these answers are very interesting. I was planning on these guys to be great bodyguards and really strong combatants to civilian targets. Dunno if there is going to be a lot of military grade stuff lying around either. $\endgroup$ – OneSurvivor Nov 5 '17 at 1:12

12 Answers 12

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Mantis Shrimp Claws will do quite nicely.

Carbon is extremely abundance on Earth's surface and presumably any other Earth-like planet that the author may be working on. Given the many forms that carbon can take from the ultra soft graphite to the ultra hard diamond, it should be able to satisfy your needs.

Characteristics of good armor

The element(s) that form this armor are important but the construction/organization of the elements are far more important. If the armor is too rigid, it will shatter. If it's not hard enough, then the bullet will pass through. (An example of a behavior we don't want is spider silk. True, it's stronger than steel at that scale but it's also super stretchy. Stopping a bullet on the other side of the target isn't very useful.) The goal will be to spread the bullet's kinetic energy over a large enough period that the armor plate can handle it.

Mantis Shrimp armor

Mantis shrimp have ridiculously resilient armor. They have to since they hit harder than most anything else in the animal kingdom. Note the many layers of fibers at slight angles to each other. In this configuration, penetrations that break through between two parallel fibers (this is the weakest configuration) runs into the next layer below that is oriented more towards the longitudinal orientation (which is far far stronger). At every layer, the bullet is forced to expend lots of energy breaking the bonds of the fibers along their strongest axis.

Fast Replacement Armor

Requirement 4 is the most interesting. Growing plates like a turtle shell would certainly be effective from an armor perspective however, these don't grow quickly. Human skin doesn't offer any kind of armor capabilities but it does grow very quickly. Skin wounds can heal in a month or less (depending on various factors). Clearly we need something that will grow fast and ideally is always growing. If the armor is damaged, we don't want to have to keep carrying it around for longer than we have to. Lots of animals have disposable "armor" to one degree or another. Humans have their skin. Porcupines have their quills (which are replaced). Snakes, lizards, crabs and lobsters have their skins.

Let's assume this creature is a carnivore so that it can afford the higher metabolic costs of replacing all it's armor in a month or so. Perhaps as a way to reduce this metabolic load, the creature swallows the old armor scales which are then broken down into basic components for the armor-building cells to use.

Alternatively, the outer layers can just flake off after exposure to oxygen for some period. This gives the armor a natural decay rate and prevents the armor from getting too thick. Natural variation in the armor breakdown proteins could lead to some creatures with thicker or thinner armor than others. Hey cool! We just invented a way to get heavy and light versions of the same creature suited for different battlefield duties without having to breed different versions. Win!

Yeah, but how good is it?

Mantis shrimp 'fists' are known to withstand 4 gigapascals. This is about 40k bar or 1/90th the pressures in Earth's core. Dang.

I'm going to assume a NATO 5.56x45mm round. It's super common and well understood. With a muzzle velocity of 990m/s. Kinetic Energy is:

$$KE=\frac{1}{2}mv^2$$ $$\Delta E=F\parallel d$$ therefore $$F=\frac{\Delta E}{d}$$ Assume $E_0=0\text{ J}$

therefore $F=\frac{mv^2}{2d}$. $$P=\frac{F}{A}$$ $$A=\pi r^2$$ therefore $P=\frac{mv^2}{2d\pi r^2}$,

where $P$ is the pressure in Pascals, $m$ is the mass of the bullet in kilograms, $v$ is the muzzle velocity in meters per second, $d$ is the distance the bullet travels in meters, and $r$ is the radius of the bullet. With all that, we get that the pressure exerted on the armor is

$$\frac{.004\cdot990^2}{2\cdot d\cdot\pi\cdot0.00285^2}\approx \frac{77}{d}\text{ Megapascals (MPa)}$$

The farther away the gun, the less pressure exerted. With simple algebra, we can find that you would have to fire from just under 2 cm away to reach a breaking pressure of 4 GPa.

This is only an approximation since these calculations don't include angle of impact, thickness of armor, ablation effects, the liquid characteristics of metals at high speeds and small time frames, tungsten at high speeds, possible pyrophoric effects, and so on.

Turn it up to 11

So far we've been talking about common mantis shrimp armor. Cool. Let's turn it up to 11 by replacing whatever carbon/calcium materials are in their fists for carbon nanotubes. Given that the theoretical maximum for carbon nanotubes is approximately 100gpa (about 25x our baseline), replacing a substantial portion of the default fist matrix should yield impressive strength gains. I'm no materials engineer so I can't prove it. I only play one on the internet.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – HDE 226868 Nov 3 '17 at 3:34
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    $\begingroup$ "Human skin doesn't offer any kind of armor capabilities" I would feel quite vulnerable without my skin... $\endgroup$ – Daniel Darabos Nov 3 '17 at 10:08
  • $\begingroup$ While I trust your numbers and the math you did, I'm not sure about the conclusion... I have the strong suspicion that if you would actually fire a bullet at some Mantis Shrimp clubs (they are some centimeters long) they would simply shatter in a million pieces. Meaning that all the ignored variables like impulse, etc. ... play a major role in this. $\endgroup$ – fgysin Nov 16 '17 at 14:56
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    $\begingroup$ As this is all speculation, you are free to disagree as you please :) However, if you do happen to write a simulation for armor of this type incorporating ballistics effects, chemical bond strength, projectile/armor combustion, fluid deformation of high speed metal, potential pyrophoric effects and all the other variables required to do this accurately, I'd love to see what you come up with. $\endgroup$ – Green Nov 16 '17 at 15:33
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    $\begingroup$ If you fired at an actual mantis shrimp club it would undoubtedly shatter. The armor is just far too thin. But nothing stops you from making your own on the same principles but a few centimeters thick. $\endgroup$ – Drunken Code Monkey Jan 5 '18 at 0:36
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Good News Everyone! We already have a natural armor that can resist bullets.

I introduce to you the humble Abalone.

A Bullet Stopper

After millions of birds pecking at these things trying to get to the soft, gooey interior, they have evolved the best shell currently known to man. These shells have literally evolved to withstand concentrated, rapid forces.

If you crush and glue 2-4 shells together, you can make a little 1-inch super shell. This super shell can literally withstand bullets. It accomplishes this by having a highly ordered shell. If you zoom in, it looks like a bunch of little bricks in a brick wall, whereas other shells look like a bunch of little sticks randomly glued together.

This is still the subject of research of many material scientists, but I do know that the NSF Center for High Voltage/Temperature Materials and Structures, which draws members from Denver University, University of Connecticut, and University of Illinois at Urbana-Champagne, and Michigan Tech., are actively looking into this. Sadly, I don't have the exact data right now, and I don't think they've published any yet, but unofficial reports show these layered shells do withstand bullets from low-to-medium power firearms, and shows promise of even withstanding higher-powered firearms.

When they do break, they crack, but mechanisms similar to bone re-growth could be used to help repair it. Alternatively, a creature could have "abalone-inspired" plates which regrow after a particular amount of time.

While these are not currently made from the air, the elements needed are biologically available. These little mollusks may be the key to better bullet-proofing in the very near future.

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    $\begingroup$ I upvoted this simply because there are substances created in the natural world that work on the nano and molecular levels to achieve results beyond our beliefs and capabilities. $\endgroup$ – anon Oct 30 '17 at 19:45
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    $\begingroup$ Once again, the best answers to "how in the world could this possibly work" are "here, nature's done it already!" $\endgroup$ – Cort Ammon Oct 30 '17 at 19:48
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    $\begingroup$ Here is more on the abalone shell strength 'Nature's armor' could help engineers design stronger materials $\endgroup$ – Justin Thyme Oct 30 '17 at 20:08
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    $\begingroup$ @OneSurvivor Sadly, this is still the subject of research (which I believe is still unpublished) of the NSF-sponsored HVT Group. The trick here is to figure out either harvest it or reproduce it for human applications. $\endgroup$ – PipperChip Oct 30 '17 at 20:35
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    $\begingroup$ 'If you crush and glue 2-4 shells together, this super shell can literally withstand bullets.' This is a pretty astonishing claim and I don't see any reference to it in either of the links in the answer, can you cite it? $\endgroup$ – Catgut Oct 30 '17 at 20:36
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All of the answers that I've skimmed over are really good, but there's one fatal flaw to all of them. When I was in Army Basic training in 1997, we used the M16A2. The standard round for that was the green tipped armor piercing 5.56mm round.

For demonstrations purposes, the drill sergeants filled an empty steel ammo box full of water and shot a hole completely through it, including the back side.

https://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO#United_States

I'm assuming that "US standard issue rifle" means US military.

Things have changed since 1997, but I would assume that current military weapon standards retain the armor piercing capability. I'm not saying the M16 rounds or the rifle rounds of today can pierce a tank, but could pierce a vest.

https://en.wikipedia.org/wiki/M16_rifle#Terminal_ballistics

That link states multiple penetrations stats for the M16, with the (I personally think) most interesting stat being the "31 layers of kevlar". This doesn't state whether the test were using the armor piercing round, but it seems like it.

As good as even @Green's answer is, I don't think even the mantis shrimp claw could withstand armor piercing bullets.

The carbon nano tubules might have a fighting chance, though. I remember a book entitled "Sentenced to Prism" that talked about silicon based life forms of a massive variety. A quick Google says Alan Dean Foster is the author. One of my favorites, and it talks about organisms that can take a beating, lasers, and I think even bullets with a variety of defenses for each. It's a great read, so you'll probably enjoy "researching" that book some.

Good luck with all your research and I hope you find something worth using!

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    $\begingroup$ You make a good point and I don't have a good defense for it other than testing artificial mantis shrimp armor against tungsten core armor piercing. I'm aware that there some scaling issues at play here where a tiny strong thing may not be so strong at larger scales. $\endgroup$ – Green Oct 30 '17 at 21:58
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    $\begingroup$ Armor piercing bullets are made to pierce armor. The armor was there first, and the bullets came later, specifically to penetrate the armor. $\endgroup$ – Rissiepit Oct 31 '17 at 6:06
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    $\begingroup$ @Rissiepit: Specifically to penetrate that particular armour. The bullet's advantage only lasts till someone invents an armour that can withstand that particular bullet. And the cycle continues... $\endgroup$ – nzaman Oct 31 '17 at 12:24
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    $\begingroup$ Okay, commenting for @Mike: Interesting foray into outdated and incorrect terminal ballistics of the 5.56 cartridge by a doubter. Water is not an exemplar for anything or anybody but demonstrations to greenhorns. eg: don't hide behind water filled ammo cans. Indeed Steve Martin's oil cans might be somewhat more preferable. Green tip is not armor piercing though I'll grant you that common misconception. Body armor is classified by threat levels that generally map to cartridges as well as the amount of tolerable body damage. (cont.) $\endgroup$ – OneSurvivor Oct 31 '17 at 18:55
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    $\begingroup$ Part of the reason I went with 20 yr old info is because even then, the US military used armor piercing rounds. (The Wiki on the 5.56 lists several variety of armor piercing rounds with the green tip. Green tip can mean different things for different rounds/brands.) Also, the MythBusters found out that water can stop bullets effectively, and that the higher the muzzle velocity, the better water works. Google: "mythbusters shooting into water". But, yes, the water was mostly there to immediately and easily show the hole in the ammo can on both sides. $\endgroup$ – computercarguy Oct 31 '17 at 20:36
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Carbon nano tubes!
If the atmosphere has a lot of CO2, say from volcanoes, pollution, or whatever, you could suck it in, break the carbon oxygen bond, keep the carbon and release the O2. Then you take those carbon atoms and string them along into long chains, form those strands into tubes, and now you have a carbon nano tube. Then you can take lots of those tubes and weave them into a thread. Take lots of threads and weave them into a very strong cloth.
It probably wouldn't be all that fast though.

For a creature to do it using biological means isn't all that crazy, as lots of bacteria use chemical processes to break molecular bonds, and instead of weaving the fibers into a cloth it could embed them into it's skin to toughen it. This would be a gradual process that starts at birth as skin cells form, and the older it got the thicker and tougher the skin would grow.
If the skin is damaged, new tissue would grow to heal the wound, which would have new carbon nano tube strands, and the scar tissue would be tougher than the original skin.
Our body armor has reinforcing ceramic plates in key locations to stop blows to soft organs.
This creature could easily do the same thing with bones like ribs to absorb and shield the kinetic energy. They would also be self healing if a bullet is able to damage one.

One interesting part of this is that if the creature acquires it's carbon from CO2 it breaths in, it could potentially recycle the CO2 from it's own exhalations, which means that it could potentially hold it's breath for a very long time.

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    $\begingroup$ "...the scar tissue would be tougher than the original skin." This seems like a very interesting story element, as if you don't kill one of these things, they only get tougher. They may even injure themselves to get the coveted scars. Interesting idea. $\endgroup$ – OneSurvivor Oct 30 '17 at 19:24
  • $\begingroup$ Carbon nano tubes are indeed being investigated for usage in vests though one problem with this is; vests are made of 2 components, a cloth capable of catching a bullet without it penetrating and a ceramic plate capable of dissipating the energy over a larger surface area. Your carbon fibre cloth does not have the properties needed to dissipate the kinetic energy. $\endgroup$ – anon Oct 30 '17 at 19:50
  • $\begingroup$ @anon but the creature might have some tough bones, like ribs, that could act like ceramic plates, and be self healing if a bullet damages one. $\endgroup$ – AndyD273 Oct 30 '17 at 20:43
  • $\begingroup$ Then that needs to be part of your answer, because sure your nano tube skin is stopping penetration but the force of impact (blunt trauma)on a kidney or brain is still just as fatal. IE It hasn't stopped the kinetic energy of the bullet $\endgroup$ – anon Oct 30 '17 at 20:46
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    $\begingroup$ ...the scar tissue would be tougher than the original skin. even humans do this on a regular basis. Check out how Thai boxing professionals train :) They repeatedly kick their shinbones against progressively harder materials in order to create microfractures that heal into a much tougher bone. They can even kick wooden baseball bats into two! $\endgroup$ – Juha Untinen Oct 31 '17 at 12:14
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I can't satisfy all the requirements of the OP, but here is a partial solution. A naturally occurring fiber that is capable of being used to create a bullet stopping armor is SILK

Silk has a very high tensile strength and is elastic. It is naturally spun by silk-worms. In the real world, the silk fibers sourced from silk-worms needs to be spun into fabric, which is then tailored into a protective bullet-proof vest. So called Dragon Silk is already under development for bullet proof vests for US military.

The use of silk based fabric for armor is actually old. However, with modern genetic engineering technology, the genetically modified silk-worms are capable of stopping bullets.

There are two ways in which this would work on a 'natural' setting:

  1. Symbiosis : The creature has a symbiotic relation with silk-worms, that spin several layers of their silk fiber around the creature. This provides the creature with a armor. The worms do this continuously, so the armor is regenerative even if it is partially damaged.

  2. Stem cells : The genes of the creature as spliced with that of silk-worms such that the skin of the creature creates this silk-fiber. Over time the skin of the creature acquires several layers of this fiber that gets inter-meshed, to serve as an armor that protects it.

The atmosphere requirement isn't pertinent here. But the silk based armor is a 'realistic' technology that is currently under development and therefore a plausible option.

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  • $\begingroup$ Nope, textile based armor performs poorly vs sharp pointed bullets like 5.56 ir 7.62. That's why military vests that have to counter those use hardened steel or ceramic plates, kevlar in those only stops shrapnel. The silk would only provide protection vs blunt pistol rounds, but then again there are hides that can resist certain pistols cartridges. $\endgroup$ – Nick Dzink Nov 4 '17 at 23:34
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Do you need armour that stops a bullet like a steel plate, or armour that stops death even if there is some injury, because if the latter you just need a high-tensile fabric 'skin' that has enough give to it to absorb impact.

Old Mongol horsemen used silk shirts to defend against arrows - they worked even though the arrow would hit and stab into them, it wouldn't penetrate the silk and the arrow wouldn't do much damage. Kevlar works in much the same way - the fabric doesn't get cut through and the bullets cause only bruising damage.

So a creature with tough, but relatively loose skin with a good layer of blubber behind that (kevlar vests flex by up to 4cm apparently) would be quite safe, to a point.

An alternative would be a creature with a subdermal layer of fat made of custard (or something similarly non-newtonian). Always beware the custard-monsters!

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All answers on this question are measuring against a direct perpendicular impact. I would expect a sticky coated armor that would present a geometrically angled surface might have some effect on slowing down a rifle round enough that there would be little penetration. Try to develop an completely inelastic collision with angles to create lateral forces and friction to reduce the penetration vector. It might take a period of time for the atmosphere to re-coat the armor with this adhesive goo or to realign the geometric plates.

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  • $\begingroup$ Interesting. Will probably incorporate this into the final "build". $\endgroup$ – OneSurvivor Nov 2 '17 at 20:36
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How about steel? Steel is Iron and Carbon, in theory an engineered lifeform in a reasonably metal rich environment could have a bark-like growth of steel in place of its dermis, this material grows from the inside like tree bark, being relatively inert in it's outer layers. Life on Earth can already suffer from excessive Iron, it caused a disorder called hemochromatosis where excess Iron from dietary intake mineralises in the blood causing organ damage. This creature does the same thing, re-mineralising ingested Iron but only in it's skin and on purpose, this creature would have enhanced Iron absorption mechanisms and possibly also use an Iron, rather than Calcium, based skeletal chemistry to simplify its Iron absorption pathways (most vertebrate life on Earth has digestive chemistry geared to absorb Calcium preferential to Iron). Sorry I don't know how thickly it would need to be plated in steel-bark but it would be a continually growing and incrementally shedding armoured shell.

I feel like I missed something so let me know if you need anything else.

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    $\begingroup$ There is actually a kind of snail called the scaly-foot gastropod, which defends itself with iron armor. More specifically, it has three layers of armor - iron on the outside, calcium on the inside, and squishy organic matter in between. It lives near deep-sea hydrothermal vents and uses the iron sulfides spewing out from inside the earth to construct its shell, with the help of symbiotic bacteria. However, this option is less viable for most animals that don't live in exotic, metal-rich environments. $\endgroup$ – IndigoFenix Nov 1 '17 at 13:39
  • $\begingroup$ @IndigoFenix Cool, yeah I have assumed some exoticism in chemistry, partly because of related questions mentioning that the creature is, potentially heavily, genetically engineered. $\endgroup$ – Ash Nov 1 '17 at 13:47
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Building on the idea of an earlier answer, multi layer armour.

Layer 1: outermost; a tough flexible sheet, possibly some sort of strong leather, that can bend slightly. Protects the inner layers from superficial damage; allows some freedom of movement.

Layer 2: first inner layer. Some gel like substance. Slows down the bullet as it passes through and disperses the energy of the bullet hitting the third area.

Layer 3: plates of ceramic composite (bone or chitin). Plates instead of a single large plate to allow wearer to move. The bullet will shatter the plate it hits, but the energy of the shards and the reaction on the bullet will be absorbed by layers 2 and 4.

Layer 4: a thinner version of layer 2. Prevents shrapnel from forcing its way through the last layer and injuring the wearer.

Layer 5: Silk or leader inner armour. Absorbs any remaining momentum and acts as a framework on which to build the other layers.

The problem is that any armour thick enough to stand up to an automatic rifle is going to be too heavy to wear. What this method does is to treat the incoming bullet as the tip of a wave and try to refract as much of it away as possible. Deflection instead of blocking. It probably won't stand up to a shot at centre mass, though.

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Graphene

Lee and colleagues devised a new miniature ballistics test to test graphene’s mettle. They used a laser pulse to superheat gold filaments until they vaporised, acting like gunpowder to fire a micrometre-size glass bullet into 10 to 100 sheets of graphene at 3 kilometres per second – about three times the speed of a bullet fired from an M16 rifle.

The team found that graphene sheets dissipate this kinetic energy by stretching into a cone shape at the bullet’s impact point, and then by cracking outward radially. The cracks are one weakness of single-layer graphene, Lee says, but it nevertheless performs twice as well as Kevlar and withstands 10 times the kinetic energy that steel can. Using multiple layers of graphene or incorporating it into a composite structure could keep the cracks from spreading, too.

Researchers have been studying graphene as armour for some time, but Lee’s is the first paper to describe just how the material absorbs kinetic energy. Sound waves travel three times faster through graphene than they do through steel, which means material far beyond the impact point can quickly absorb and dissipate its energy – effectively slowing the projectile down and helping prevent its penetration. What’s more, the microbullet methods Lee developed could be used to study other high-performance materials in extreme conditions.

https://www.newscientist.com/article/dn26626-bulletproof-graphene-makes-ultra-strong-body-armour/

Better yet, it is self-repairing, too! Just add free carbon atoms and they will snap into place in a broken lattice.

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Armadillos.

There is at least one documented case of an armadillo being resilient to pistol shots:

https://www.huffingtonpost.com/entry/texas-armadillo-shooting_us_59838ae2e4b08b75dcc5f622

(.38 shots fired at armadillo, animal was able to walk away, shooter got severely injured by ricochet).

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  • $\begingroup$ The highest energy .38 round I could find still has less than half the muzzle energy of the 5.56×45 mm NATO. I'm afraid there's a pretty big difference between pistol and intermediate rifle rounds. $\endgroup$ – timuzhti Nov 2 '17 at 14:16
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An amour with many layers. As each layer will reduce bullet speed. There are examples in the animal kingdom where animals grow layers of hard shells or use natural resources like sand, etc. to create armors which can stop even a bullet.

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  • $\begingroup$ Do you have an example of such an animal? A source you could cite? That would definitely improve your post. At least I have never heard of an animal that survived getting shot. $\endgroup$ – Secespitus Dec 7 '17 at 17:12

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