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The trolls in my story have a very thick and nigh impenetrable skin. In the time before guns, you would need a very sharp high-grade steel blade to breach the skin and, even then, after a few hits, the blade would be dulled significantly.

The trolls are very thick-skinned and the skin resembles a stone like material like thick callouses. It is covered in layers of moss and lichen. The trolls are turned to stone during the day and they don't really care about how clean they are. The trolls will be roughly 4-5 meters high (10-13 feet)

I did some research and the breaking point of a claymore is 2624 newtons.

A 9mm bullet weighs 7.45 grams and will travel roughly at 310 meters per second. Then 0.00745 kg * 310m/s gives us an impact force of 358 ish newtons.

A baseball hitter can hit hit a baseball with much greater forces than the breaking point of a claymore.

So let's put a guess at penetration at roughly 2200 newtons needed with a sharp edge.

So a sword seems a lot more effective than a 9mm gunshot. Would a skin that can withstand ordinary sword slash and jab attacks from a skilled swordsman be able to resist bullets from modern day firearms? My basic lookups seem to indicate so. If so, what caliber would be needed for penetration?

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    $\begingroup$ Do your trolls' hides resist blades because they've evolved to resist blades (making them similar to a modern stab vest), or just through sheer thickness? $\endgroup$
    – F1Krazy
    Feb 4, 2019 at 12:52
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    $\begingroup$ "Then 0.00745 kg * 310m/s gives us a force of 23newton." $\mathbf{F} \neq m \mathbf{v}$, so this number is wrong. You've actually calculated the momentum of the bullet. The force applied is going to depend on how quickly the bullet stops and will probably be far higher than 23 N. The pressure will be even higher, since the bullet has a small area. But determining what the bullet penetrates is a very complex problem that depends on many variables. It's probably much easier to look at actual experimental data collected from shooting rocks. $\endgroup$
    – el duderino
    Feb 4, 2019 at 13:45
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    $\begingroup$ Our world has armor-piercing bullets - so unless your troll hide is stronger than a modern flak jacket, the troll will die fairly quickly. Worse, as you toughen the hide to make it more bullet proof, you also lower its flexibility (bullet proof vests don't flex easily). There's a price to pay to being modern-day bullet proof. $\endgroup$
    – JBH
    Feb 4, 2019 at 14:43
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    $\begingroup$ And @elduderino is right, your math is wrong. That bullet should have at least 1,000 N of force. Review this page for a nice, short tutorial about finding the force of a bullet. $\endgroup$
    – JBH
    Feb 4, 2019 at 14:50
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    $\begingroup$ Impact force, F=1/2mv^2. That is, half the mass times the square of velocity. You don't really need all that big a bullet, you just need a fast one. A NATO 5.56 would likely penetrate just fine, and a .17 Remington Fireball would be even better, though a larger round might produce more tissue damage. $\endgroup$
    – Monty Wild ♦
    Feb 5, 2019 at 11:24

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Your trolls seem to be made similar to rhinoceros or elephant skin. Normal ordinance (like small caliber ammo) have usually poor penetration against these (they can cause bleeding but won't penetrate), but there is ammo designed specifically to hunt big pieces that will penetrate the skin and usually with terrible effects (as the skin is so thick, the bullets tend to tumble and/or break, causing horrible internal wounds).

And if your trolls have a skin thick enough to surpass the elephant... NTW-20

Big guns for big problems present the Denel NTW-20 antimaterial rifle. With a weight of more than 30kg it is not exactly portable but can shoot bullets up to 20mm diameter x 110mm length with a force of 40kJ at a distance of 1.8km. This is made to penetrate walls and lightly armored vehicles (that's why it's called anti-material). We guarantee no troll will withstand a well placed shot or we give your money back.

EDIT: As I've seen in your clarifications that the idea is early defense in case trolls that we haven't seen for 1000 years awake suddenly, my guess is that not everyone has or can afford an Anti-Material rifle. So, as an early (and let's be honest, more practical) defense...

CZ-550

All the Game introduces the CZ-550 big game hunting gun. Chambered with a .375 magnum H&H that produces up to 6kJ of force, this gun manages to penetrate the skin of the biggest game while retaining a manageable recoil at a reasonable distance (it retains good accuracy up to 800m, although usually is shot between 300 and 500). It is also you can find in almost every major city of the planet (although not in great quantities), even in gun-regulated Europe (that frowns on anything remotely militar, but loves big bucks customers that love hunting). Buy it today and we will throw a sample of our new jacketed "no expansion" cartridge!

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    $\begingroup$ We have weapon company representative here now? Good answer though. Hunting rifle for big game have absurd firepower. If memory serve, post-WW1 Germany was prohibited to developp any anti-tank weapon, so they designed a "hunting rifle" that was incidentally able to pierce light armour-plating. $\endgroup$
    – Nyakouai
    Feb 4, 2019 at 14:50
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    $\begingroup$ "Big guns for big problems" I like that. $\endgroup$
    – MongoTheGeek
    Feb 4, 2019 at 18:17
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    $\begingroup$ @Nyakouai: Whenever I have to present a real world thing for fantasy applications, I cannot but think I feel a bit like a whatever vendor, and my inner seller kicks in :) Yes, Hunting rifles and revolvers are usually overpowered. My second choice after the AM rifle was a .50 casull revolver $\endgroup$
    – Stormbolter
    Feb 5, 2019 at 8:40
  • $\begingroup$ Love the update. Big game hunter coming out to chase the big bad. $\endgroup$
    – Tschallacka
    Feb 5, 2019 at 17:05
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While Stormbolter gave a concise and effective answer to your question, I'd like to add some things for you to think about.

What I'm guessing from your question is humans started fighting trolls using swords. But even this method, while it manages to pierce the troll skin is: a) costly, since it dulls the sword extremely fast, b) probably dangerous. Hence, humans may or may not improve firearms technology and you're wondering if it would be effective and worth the time for them (again, this is purely a guess).

I'm no military historian, but I believe cannon predate smaller firearms (Edit: by about a hundred years in Europe. We indeed saw the appeal of handheld cannon pretty quickly), simply because they're easier to conceive. Miniaturizing something is no trivial task, much so if you're still figuring how to properly contain the detonation/deflagration of the powder.

From an evolutionnary point of view, humans would have first developped big cannons which would be extremely effective against pretty much anything, as long as they touch.

But let's say your magical creature is somehow impervious to cannonball. Even if your munition does not pierce the troll hide, the mere resulting shock will turns its innard to pulp, and most living organisms end up pretty much dead when that happens.

You would probably observe tactics such as seen in history, with artillery thinning the ennemy lines, before the infantry engage in the dirty work. Since each shot can probably kill a troll on the spot (once again, as long as it touch), humans will work to improve the technology, and eventually come to big hunting rifle.

The last paragraph is of course only pertinent if trolls band up in large groups to attack humans. But defending settlement from troll attack would still be a lot easier if you can shoot the thing before it even become remotely menacing. And then, you follow the same path as explained before, and you have boomsticks to deal with trolls.

Edit: Given your comment, troll as a menace that has "awoken" from the past, it boils down to what people said. Small caliber, unlikely to pierce. Special ammo, depends. Bigger guns will surely put a hole in your trolls though. Local forces may been variably equipped, but even small police department in my country have at least one shotgun or a SMG lying around. Not much to fight a full scale invasion, probably enough to repel a single troll until army get there.

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  • $\begingroup$ The setting is more that trolls have vainished for roughly 1000 years and suddenly re-appear through an event, waking up as it is. And yes, working through troll hide was a dangerous thing and people would go for the legs to immobilize it to finish it off when it's grounded. Now when the trolls wake up, I'm trying to figure out how modern day defenses would work against it since it would be a "new" foe. How well would a police gun work, what would the military drag in to combat the 3-4 meter very movable nigh groundable foe. Would a tank be mobile enough to counter a troll? $\endgroup$
    – Tschallacka
    Feb 4, 2019 at 16:18
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    $\begingroup$ Depends of your troll, where you fight them, etc, etc. A tank is (for most people) surprisingly "agile". They accelerate way faster than a car, though they don't have near the same maximum speed. I think military would adapt quickly and arm themselves in consequence. Squads with heavy rifle to deal with trolls, vehicles with heavy weaponnery (and I'm talking machine guns, not even cannon), sniper with anti material rifle... I'm not military, but I wouldn't worry too much about our capacity to kill things. $\endgroup$
    – Nyakouai
    Feb 4, 2019 at 16:25
  • $\begingroup$ Every police car has a shotgun in the trunk. There should be no shortage. $\endgroup$
    – Andrey
    Feb 4, 2019 at 19:48
  • $\begingroup$ @Andrey strongly depends of the country ^^ $\endgroup$
    – Nyakouai
    Feb 4, 2019 at 19:53
  • $\begingroup$ In Desert Storm (Iraq 1) the A 10 Warthog's 25mm canon, designed for anti-tank operations, was employed in antipersonnel operations. I suspect your trolls would only fare marginally better than a human. $\endgroup$
    – pojo-guy
    Feb 4, 2019 at 22:10
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Sandbag penetration should be roughly equivalent to what you need. Your description of the hides, "a stone like material" and "turned to stone during the day" sounds to me like something made of "rock" just not in a solid slab (sand), held together by something flexible (bag) to allow some movement ... a Sandbag skin.

It should take very little research in to the military applications of sandbags as defense to find out just how thick you want the skin to be to get the type of defense your Trolls would need or have.

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  • $\begingroup$ That's a really neat idea. I might do something with that. $\endgroup$
    – Tschallacka
    Feb 5, 2019 at 17:03

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