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?