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I've been designing a race of humanoids with a thick chitinous arthropodal carapace resilient to small-calibre gunfire and physical trauma that would cripple a normal human being. This carapace is similar to that of the Iron Snail's, which has a tri-layered shell protecting the snail from predators and the extreme environment it lives in. The inner layer of their shell is made of aragonite, a form of calcium carbonate commonly found both in the shells of molluscs and in various corals. This helps dissipate heat, saving the snail from a slow, volcanic-induced simmer.

The middle layer is equivalent to the organic outer layer also found in other gastropods, and is also the thickest of the three. This layer appears to act as padding absorb the mechanical strain and energy generated by a squeezing attack making the shell much tougher, and harder to break.

But most impressive of all is the outer layer. It is made of iron sulphides, in particular pyrite and greigite (which range from 6-6.5 and 4-4.5 on the Mohs scale hardness). Greigite nanoparticles covering the outer shell mainly serve as a shock absorbent. When the snail’s shell is subjected to a high pressure, the shell cracks around the particles, absorbing energy and blunting and deforming the predators’ claws. The shell thus becomes littered with thousands of microcracks that not only absorb the shock, but also prevent large cracks from forming on the shell which would have been fatal for the snail.

So, when everything is said and done, how effective would this carapace be at stopping small-calibre bullets and how would it fare against high-calibre gunfire?

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    $\begingroup$ Relevant: Watch 1"-thick granite (countertop) get shattered by a single AR15 bullet. Look how heavy the granite is, too. Bullets have an enormous amount of kinetic energy that your lifeform must absorb in a fraction of a second. $\endgroup$ – user535733 Aug 25 '18 at 15:56
  • $\begingroup$ this is going to depend a lot on the size of the animal and the thickness of the armor. most shells are so thin it does not matter what they are made of compared to bullets. $\endgroup$ – John Aug 25 '18 at 18:08
  • $\begingroup$ Even if the bullet fails to penetrate, the kinetic energy could still rupture organs if the layer isn't thick enough. $\endgroup$ – Richard Smith Aug 25 '18 at 19:28
  • $\begingroup$ I'm going to give you a +1 just for introducing me to the iron snail. $\endgroup$ – Era Aug 26 '18 at 1:15
  • $\begingroup$ If iron sulfide were that effective at resisting small arms fire, we'd already be using it in personal armor plating. $\endgroup$ – RonJohn Sep 21 '18 at 19:09
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I think the answer is "Very effective". The animal you really want to have a look at for this purpose is the Mantis Shrimp.

Here's the really critical bit: "Further inside, highly organized layers of chitin fibers act as shock absorbers. The fibers are arranged in a helical structure to slow down the spreading of cracks. Here's the trick: the helix forces the cracks to constantly change direction, which disperses their energy and quickly stops them from propagating."

This link has less detail, but some cool videos.

"Scientists are already trying to harness the durability of oyster shells to develop bullet proof windshields of military vehicles, and now they’re turning to the mantis shrimp’s shell in designing the rest of the Humvee."

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The carapace might be effective at stopping small caliber fire, but that is not enough to ensure survival. Shot thrown from a sling (one of the oldest weapons known to man) was quite capable of killing an armored human, without leaving a mark. The impact against the armor was sufficient to cause internal injuries to organs in close proximity to the impact site, particularly to the brain and got organs.

Edit This is an older article that discusses massive internal injuries without external bruising. examples incluse a 5 year old child who was run over by a hansom cab (horse drawn carriage), who died from severe contusion and laceration to the lungs, but there was no skin or rib injury, and the loser of an altercation who died after having most of his ribs fractured, and his heart ruptured, and his liver, stomach, kidneys, and spleen all being lacerated, but with no marks of violence visible except for a bruise and laceration on the scalp.

ncbi.nlm.nih.gov/pmc/articles/PMC2317961/?page=1

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    $\begingroup$ "Shot thrown from a sling (one of the oldest weapons known to man) was quite capable of killing an armored human, without leaving a mark." Citation? $\endgroup$ – RonJohn Aug 26 '18 at 4:12
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    $\begingroup$ That doesn't matter. (Note that the part I question is "without leaving a mark", not that a sling can kill someone.) $\endgroup$ – RonJohn Aug 26 '18 at 4:35
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    $\begingroup$ Through something solid like a helmet the force is disputed over enough area to not cause surface bruising, but the energy is still sufficient to cause internal injuries. This is the same principle used by more modern thugs such as Union enforcers and rogue police officers to beat someone, using a telephone book as a buffer, leaving internal injuries but no visible bruising. Looking for a medical reference on traumatic injuries without bruising. $\endgroup$ – pojo-guy Aug 26 '18 at 9:52
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    $\begingroup$ Is a general article attesting to traumatic internal injuries without external bruising sufficient, or do I need to specifically find a citation regarding slings? ncbi.nlm.nih.gov/pmc/articles/PMC2317961/?page=1 $\endgroup$ – pojo-guy Aug 26 '18 at 10:11
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    $\begingroup$ It's a perfect article. +1. You should add it -- with a relevant quote -- to the answer. $\endgroup$ – RonJohn Aug 26 '18 at 12:25
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Is your carapace tougher than 3/8" thick steel plate?

5.56x45 M855 at 140 yards zips right through... Also, just how shatter proof can it get? IIRC from all the pyrite I've found it shatters pretty nicely. Can it withstand the energy being imparted onto such a small area?

img of bullet holes in steel plate

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  • $\begingroup$ is a 5.56 considered small calibre? penetration capabilities increase significantly when using a rifle calibre like the 5.56. A 9mm or even a .45 are way easier to stop because they are much slower. $\endgroup$ – RancidCrab Aug 26 '18 at 11:59
  • $\begingroup$ @RancidCrab - "Small arms" is basically anything up to and including .50bmg. As military calibers go, 5.56x45 (what the M16/M4 uses) is on the small end - the M855 load is a .224" diameter 62gr (~4 gram) bullet moving at just under 3000 feet/sec. 762x39 (AK round) is a 30 caliber 125grain (~8 gram) bullet moving at 2600fps, and 762NATO is a 150 grain bullet (~ 10gram) moving at 2800 fps. $\endgroup$ – ivanivan Aug 26 '18 at 14:14

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