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Usually this "indestructible" material is unnaturally strong for its weight; the armor "just exists" and doesn't go into the feasibility of such a material.

Given that the material isn't "hard to obtain" (anybody is capable of obtaining the armor) and passive (no type of "energy" should be required for it to work), how might I explain that nobody simply creates bullets or other offensive items that are made out of the same material in order to overcome the armor?

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Tensile versus compressive strength (or strength along different axis) could be a start. – Michael Kjörling Jan 8 at 19:46
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What kind of world you are building? What's their technology? Is there magic? How world physics differ from ours? In case of magic it can be as simple as "empathic" metal that only works in close proximity to the body, as for technological means, I don't have any good idea yet. – M i ech Jan 8 at 19:57
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@LorenPechtel I think the idea is that if it's strong and indestructible, it might be able to pierce itself. Like the adamantium bullets that punch through Wolverines skull at the end of the Origins movie. – AndyD273 Jan 9 at 5:15
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Won't that give you the Chinese parable about impenetrable shield and the all piercing spear (paradox)? – user6760 Jan 9 at 8:57
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Sounds like General Product's hulls from Niven's Known Space universe. Niven worked around it simply by having the manufacturer produce the hulls in four standard shapes/sizes. Since the hulls are indestructible, the cannot be broken down and repurposed as bullets. (in one of the novels a group of humans did discover the secret of the hulls and found a way to break them down, but it's useless, once broken the hulls disintegrate to atomic vapor). – slebetman Jan 11 at 3:35

16 Answers 16

Make the material flexible.

Consider Kevlar: it's an ultra-strong fiber that can be woven into sheets of highly tear-resistant fabric. Thick, interlocked layers of this fabric are used to spread the force of a bullet out over a large area. Using it to make a weapon just doesn't work: it's too light to make an effective bullet, too floppy for a long-rod penetrator, and even if you somehow make it stiff enough to look like a sword, it can't hold an edge worth mentioning. I suppose you can make a Kevlar whip, but you'd look rather silly running around a battlefield with one.

Your indestructible armor would be an "ultra-Kevlar": strong enough to resist cutting (swords) as well as tearing (bullets), but not stiff enough to make a sensible weapon.

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Problem with kevlar is, it spreads force of impact which means that direct hit with sniper rifle will break a lot of bones and ordnance heavy enough will kill wearer without problems. I assume OP wants hard shell that can protect from anything, but if my assumption is wrong, seems you win the question. – M i ech Jan 8 at 20:06
    
@Miech I was mostly thinking in scale of armored vehicles vs other armored vehicles, so in a way you are correct for scaling down to human body armor - if the material is too flexible the human can be crushed even if the armor is completely unharmed. – DoubleDouble Jan 8 at 20:16
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Still worth an upvote because it satisfies the question as stated. (Though, Indiana Jones still wouldn't look silly running around a battlefield with his Kevlar whip) ;) – DoubleDouble Jan 8 at 20:29
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@Miech the human is a limiting factor anyway - for example, currently armies don't try to make helmets that can protect from a full-size rifle round since if the helmet deflects or absorbs it without penetration, then the wearer would generally die anyway from neck trauma and concussion. Many current weapons will still incapacitate soldiers with unpenetrable plates on body and limbs, simply by impact trauma or dislocated limbs. Unobtainium material alone won't give full protection unless you put it on a powerful exoskeleton. – Peteris Jan 8 at 21:42
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so you have a "bomb-proof tank", bad guy rams it with hid "bomb-proof tank" at high speed, everybody is converted to paste. – Jasen Jan 9 at 11:42

Give it a very high strength to weight ratio.
A material that is very strong but weighs almost nothing would be lousy for any kind of weapon that relies on mass.

A bullet made of aerogel or styrofoam isn't going to do any real damage, especially if you are firing it at something as hard as steel.
At any distance the air resistance will simply rob it of all inertia and drop it out of the air.

It could be used as an unbreakable line, but as a weapon there aren't many advantages to that over normal braided steel cable.

On a side note, you could braid a space elevator with it, which would be awesome!

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Dwarf fortress adamantine is like this. It makes terrible hammers but unfortunately makes exceptional swords. – Resonating Jan 8 at 22:57
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@Resonating: And the dwarfes haven't tried to use it as a shell for some heavy but pulpy material? Or is there nothing fitting? – Deduplicator Jan 9 at 3:21
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Why not fill a thin shell of indestructible material with a heavy substance (e.g. lead)? (Of course the momentum/kinetic energy of the weapon will still limit the damage) – CodesInChaos Jan 9 at 20:19
    
@CodesInChaos Very valid point... Best I could come up with is that since the material can withstand a normal slug at those energy levels, then a jacketed round could still lack the kinetic energy to punch through, which is pretty much the same thing you said. – AndyD273 Jan 9 at 20:40
    
An "unbreakable" line (SF monomol) would slice through mere steel, tanks, battleships, etc. Very potent in offense if you can work out how to immobilise its ends! – nigel222 Jan 11 at 11:33

I second the kevlar answer, but here are soe thoughts as well.

There is such thing as non-Newtonian fluid which flows quite easily when pushed slowly but becomes almost rigid when you hit it. Google videos, they are fun - very filthy fun. Can be made at home as well.

One could theorize about a non-Newtonian cloth or plastic which is soft and elastic when moved slowly, but extremely rigid and tough when accelerated and/or deformed rapidly. Such material would be a godsend when it comes to armor: you can move, walk, run, jump all the way you want, but a bullet or club will only push, rotate, and shake you in the same pose you were before the hit. No more broken bones (but you can still suffer concussion if the projectile has enough impulse).

However, such property is useless for weapons. Make a sword that what, deforms when you swing it and stiffens when it hits the target? Would make for great supersonic whips though, but this type of armor should be immune to whipping.

The armor can still be penetrated by slowly moving knife, think Frank Herbert's Dune. Maybe physical armor (unlike force shield) could be strengthened by layers of kevlar or something.

I can also think of a sonic weapon which doesn't harm the enemy per se but binds them in place because the armor reacts to vibration as it would to a hit or fall.

But still it's a great armor overall, and no benefits for making weapons.

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It looks like similar stuff actually exists, at least in a lab. One cool thing about worlduilding is how close real life can come to what seems an unbound fantasy... – Dallaylaen Jan 8 at 23:34
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Great idea. Though, I don't like how this kind of stuff always runs under “non-Newtonian” – it's a bit like talking about non-average size people: what do you mean now, tall people, small people, fat people...? Dilatant or perhaps rheopectic fluid, that's the word you want. – leftaroundabout Jan 10 at 13:57
    
Goop like this is used in shock absorbers in high tech cars. It is stiffened electrically when it needs to be stiff, left runny for driver comfort when it doesn't. Don't think it's good enough for armour (yet?) – nigel222 Jan 11 at 11:39
    
@leftaroundabout Agreed, I wish I knew a better term. – Dallaylaen Jan 11 at 17:53

The simplest answer to the question Why don't they turn it into weapons is...

They don't need to.

In no logically consistent world will this material be as cheap as standard armor and munitions...more people will want it meaning cost will matter so making munitions out of it makes less and less sense.

At a certain point the limiting factor in an armored human scenario is the human. Your armor may be able to take a hit from a tank shell without being damaged but that is irrelevant because the human on the inside would end up liquified.

Blades would be pretty worthless unless you are talking about a heavy two hander (they are better for blunt damage than piercing/slashing).

  • Fire still works. Heat up the armor, it may still be shiny but it will have a cooked person inside.

  • Blunt damage still works. Hit an armored human with a heavy enough weight and the armor is irrelevant.

  • Electricity...still works unless your armor is an insulator.

  • Water. Armor is generally heavy, it's hard to swim with armor on...even kevlar.

  • Standard munitions (larger calibers) still work too. A .50 cal vehicle mounted machine gun can kill without actually hitting a human target, the velocity alone can snap necks and other bones...

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What about remote controlled or fully automated tanks, helicopter etc? You would need to damage the amor and not just kill the humans inside. This would probably only speed up the development of A.I – Freedo Jan 9 at 7:43
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A computer subjected to a large enough shock will break. Attacks meant to damage whatever's inside the armor will still work as before when you replace humans with robots; you just need to shake them up harder to break them. – David K Jan 9 at 21:50
    
Electricity works unless the armor is a very good insulator or a very good conductor – i.e. Faraday cage, which actually protects you better against EM (also shields microwaves) than dielectric armor. – leftaroundabout Jan 10 at 13:44

Indestructible weaponry isn't a problem for indestructible armor (or even necessarily destructible armor). The goal of armor is not to break the weapon, but rather to protect the user. For a real world analogy, consider a bullet-proof vest. If someone shoots you, the vest functions by stopping the bullet before it penetrates your body and it protects you, despite the fact that the bullet was not "destroyed" (although it may end up squished). In fact, someone could shoot an ordinary kevlar vest with an indestructible bullet, and it might get a little further into the vest as it doesn't lose energy to being squished, but the vest would still function. So, indestructible weaponry isn't even necessarily a problem for normal armor. Why would it be a problem for indestructible armor? Anything that comes at you just bounces off your armor.

Of course, it is notable that one feels a large impact if they are shot wearing a bullet-proof vest, since all that energy and momentum has to go somewhere. Such an armor would be useful to people, but it can only do so much: If a tank shoots you, there's just too much energy and momentum there for you to survive, regardless of whether the shell actually hits you - so weapons made of lesser materials can still overcome indestructible armor. That said, if you start armoring machines and buildings in this stuff (i.e. things that are less sensitive to changes in momentum), there's not much an adversary can do.

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Warning: attempt at internally-consistent handwaving. Based on answers clarifying what asker is trying to achieve, I see only one explanation: Make it susceptible to acceleration.

Normal materials are concerned with force, not acceleration, and break apart if applied force is greater than for example it's tensile strength. This material is different: no matter what force is applied, it becomes relatively fragile when it accelerates (this is "the big lie", rest is derived from this quality). Even accelerating, it can easily outperform ordinary materials, but it's the strongest when it's still or moving at constant speed, easily defeating bullets made of same material.

In practice, this means that it's great for construction, or perfect for armours and non-moving parts of weapons (like barrels, but not locks, at least not without extreme redesign) but terrible for blades, bullets or shells: 9 mm bullet weights about 100 grams and is moving at 400 m/s giving it momentum of 40 kg*m/s, same as 80 kg human moving at 0.5 m/s. During impact, bullet is subject to extreme deceleration while it transfers almost all of it's momentum to wearer, wearer on the other hand is only slightly affected - bullet becomes much more fragile than armour and shatters.

This means, that humans wearing such armour are completely immune to small arms fire, and any "indestructible" projectile lighter than target (assuming target isn't anchored in any way) will shatter and has to rely on other properties than penetration to make a kill (120mm Abrams cannon: ~8 kg projectile, 1700 m/s, after impact 80 kg wearer would be sent flying at ~150 m/s, which without liquid breathing should kill him anyway, but shell is subjected to almost 10 times higher acceleration than armour), which means that "indestructible" projectile has no advantage over normal projectile.

In practice, this remarkable property would have very interesting impact on modern battlefield: armoured infantry becomes immune to shrapnel and small arms fire, airburst bombs and artillery become completely ineffective because ground itself protects from acceleration/deceleration. The main way to kill infantry is to use projectiles and explosions intended to send them flying, killing them within armour. Infantry on the other hand is most likely equipped with some means to quickly anchor themselves to the ground/surrounding for increased protection when located in exposed or otherwise dangerous spot. As technology develops, increased weight of powered armour becomes a great boon, in addition, anchors can easily become part of armour, deploying automatically when radar detects incoming fire (same as anti-missile systems used on modern tanks). The only sure way to kill, is to use shell heavier than target, while target is in air or space.

Edit to explore effects a bit more:

Tanks become effectively immune to any regular weapons, but are not cost effective compared to infantry, infantry on the other hand is completely immune to regular weapons wielded by other infantry, this heavily changes infantry tactics:

  • use flamethrowers to cook them alive
  • hand to hand combat to subdue enemy and either physically kick out of borders, or drown/bury alive (could take a while if internal oxygen tanks become regular part of armour)
  • Weaponise whatever production process is used to shape this material
  • much higher reliance on flash-bangs and other means to incapacitate enemy while you are getting close to pry him out of his armour

Anti-tank tactics:

  • Flame-throwers (again)
  • charges (mines, bombs, precision shells and missiles) designed to flip tank around, taking it out of action without destroying it
  • Weapons intended to take out cameras, and obscure vision ports, rendering tank inoperable (smoke bombs? guided double warhead tar-and-feathers missiles?) and let infantry take it all the way down with flamthrowers.

In practice that removes tanks from battlefield, because increase in protection over infantry is not worth costs and loss in versatility and mobility, unless tanks can carry weapons that CAN damage this material while same weapons are impractical for infantry. For example, tanks could use laser/plasma weapons which are too big even for powered armours (something about fusion chamber sizes and energy requirement necessitating use of reactors?) but can penetrate this astounding material.

The way I see it, end result is a mess where infantry fights each other in melee and literally dismantles tanks if let close, while tanks rule supreme in open terrain.

Hmm, it's not actually THAT different.

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How about instead of acceleration, momentum (which is what I think you mean!) and it is relative to the gravitational frame of reference. – wizzwizz4 Jan 10 at 17:23

Make it impact sensitive

The material, like corn starch water mix becomes very hard only briefly on impact. This works well for armour as a layer of this material could be contained in the protective armour in pockets of another material or through some other containment system.

However if making bullets or blades this becomes completely useless as the moment they are not being hit against something they turn to fluid and drain away.

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I think your analogy is flawed. You are assuming that armor made from material X is unnaturally strong; so a weapon made from material X should also be unnaturally powerful(kind of like a diamond-tipped saw is required to cut diamonds).

Flaw in this logic is that the weapon and the armor are made using different goals in mind: Weapon needs to damage the enemy, not his armor. If I were to use a flamethrower or EMP on a person wearing said armor, the armor won't need to be penetrated to kill the person.

Also, as Milo Brandt said in his answer, such material will only yield indestructible weapons, not unbeatable ones.

Here's a joke question to clarify the idea:

If the black box in an airplane is never destroyed in a crash, why isn't the entire plane made of that stuff?

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Simplest answer. The real reason to not use an indestructible material for making weapons.

It is indestructible

That means that you cannot not damage it. Which means you cannot shape it, sharpen it, or generally work it. Ergo, if the raw material wasn't weapon shaped to begin with, it can never be weapon shaped.

The up shot of this is, how did it become armor shaped? Well the material could come in a variety of plate shapes, which would be woven together into some sort of plate armor.

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It might be that the material would be prohibitively expensive to make bullets out of it. Body armor that's made from a basically indestructible material would last basically forever. So, it's really expensive, but it's a one time purchase. If you could make a bullet out of that material that might not in and of itself be enough to defeat the armor. You might need to hit the same exact spot in the armor 10, 15 or 100 times to break it because of the way it's layered or hardened or whatever other way you explain it. So in that example, while technically, it could be destroyed by a projectile of the same material, it's effectively indestructible because nobody would buy bullets at $10,000 a round when there isn't even a guarantee that they will take the target out. It would be better to train people to make very precise shots with cheap ammo. It gives you more story hooks and is more interesting for things to have some weakness. Maybe there is one multi billionaire that can afford one all out attack with super bullets on one person and it bankrupts them. Weaknesses give you cool things to write about. I think that's why Superman has a weaknesses. Otherwise, where's the story? He just wins.

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If its indestructible and cheap, then turning it into a weapon would be pointless because everyone else is indestructible.

If this unobtanium is super cheap, then everyone will have it. If everyone will have it, everyone will be "indestructible". If everyone is indestructible, then no one can be killed by weapons, and if no one can be killed by weapons then weapons become pretty useless.

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It's still quite possible to kill someone, no matter how good their armor is. Consider firing someone out of an oversized cannon: even if their armor is indestructible, the acceleration will reduce them to a thin paste on the inside of their armored shell. – Mark Jan 8 at 21:22
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@Mark Perhaps, but you do not need to make your cannon out of unobtanium to do this, so it still satisfies the OP's needs – David Grinberg Jan 8 at 21:24

Suppose the process of manufacturing the armor produces a material that is lightweight and indestructible. It cannot even be dented. Suppose further that the process can be modified so that the finished product is of any size and shape we want, within reason.

I'll call this material "unobtainium."

To counter the question, why wouldn't people make weapons of unobtainium in order to overcome the armor, the response is: how would that work?

Imagine we fired a twenty-first-century anti-tank round at a soldier armored in unobtainium, and suppose the soldier is well enough braced (e.g. against a wall) so that the impact of the round does not just knock him over. What happens?

The round cannot pierce or deform the armor. It strikes the armor with great force, but the armor pushes back (Newton's laws). The reaction force may deform the anti-tank round (in fact I think it almost surely will do so) or cause it to bounce back, or both.

Now put a full jacket of unobtainium around a similar anti-tank round and fire it at the same armored soldier. What happens?

I claim that the round still exerts force on the armor and the armor still pushes back with an equal and opposite force. The difference is that this time, the unobtainium jacket prevents the anti-tank round from deforming. But it does not prevent it from bouncing off, which I believe is exactly what it would do.

An unobtainium-jacketed anti-tank round would indeed be useful as a weapon against twenty-first-century tanks: their armor is not nearly as tough as unobtainium, so a round fired with sufficient force is sure to penetrate. But if nobody is using twenty-first-century tanks in your world, the ability to pierce that armor is not worth anything.

In fact, the main thing stopping you from shooting a projectile through unobtainium armor is not the strength of your projectile, it's the inability to put enough energy and momentum into any projectile in order to overcome the armor's incredible strength. (Non-projectile weapons are even worse; the amount of energy and momentum you can put into a sword by swinging it is very limited.)

So to fight unobtainium-armored soldiers, you need to get around the armor, or find something (a force-field "blade" perhaps) that can disrupt whatever material properties give unobtainium its strength; you could shoot something at a walking soldier with enough momentum to knock him over or at least slow him down, or simply hit the armor with something that imparts so much momentum to the soldier that his bones and/or organs are injured by the acceleration, killing or disabling him. None of this implies that these weapons would be made from unobtainium.


Warning: extreme physics-geek talk below this line.

Delving into the physics a little deeper, the force on the anti-tank round has to be exerted through some distance in order to reverse the direction of the round. Since the force occurs only when the unobtainium jacket touches the unobtainium armor, and neither of these can deform, how can the force act through a distance? I suppose that when we say unobtainium cannot be deformed, we mean it cannot be deformed on a macroscopic scale (anything you could measure with a ruler or even a micrometer); but it still consists of atomic nuclei and some kind of cloud of electrons, and as these components of the jacket approach these components of the armor, there is an electric repulsive force that becomes significant only when the gap between the objects is not much more than the distance between the nuclei, and that it rapidly increases as the objects approach nearer. This rapidly-increasing force provides the "springiness" that allows two colliding pieces of unobtainium to bounce off each other.

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Use an HE round to spall off the inner lining which isn't made of unobtanium, or the unobtanium transmits the shock-wave directly to the target. The shock wave can still penetrate unless the unobtnium is unrealistically stiff. – Monty Wild Jan 11 at 23:16
    
@MontyWild That might work even if the armor is so stiff the shock wave has to move the whole suit in order to transmit anything, especially as the suit is not very heavy. The unobtainium can't absorb mechanical energy by deforming, so it might just transmit it straight through. – David K Jan 12 at 3:34

I can think of two explanations:

  1. Too heavy - Regardless of how high a material's strength to weight ratio is, it still may be impossible to make a weapon out of it that's light enough to wield.
  2. Too soft - A soft but strong material can certainly be made to absorb energy well enough to make effective armour, think Kevlar. However, that same ability could make it useless as a weapon. That's exactly why militaries tend to use jacketed ammunition, lead is to soft to penetrate armour properly.
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If it's too heavy to wield as a weapon, how do you wear it as armor? – Brythan Jan 9 at 4:50
    
Non-jacketed ammunition penetrates armor just fine -- at the energies involved, density is more important than strength. The military uses jacketed ammunition because it doesn't jam mechanical feed systems and foul gun barrels the way lead ammunition would. – Mark Jan 10 at 1:29

An exotic option that, as far as I can tell, evades many of the mentioned creative workarounds the other answers have:

If you're willing to go the way of vibranium (Captain America's shield, absorbs kinetic energy), have a material with unusual inertial properties, like a solid-state equivalent of a spinning flywheel.

Perhaps possessing huge inertia regardless of mass, possibly manifesting at greater intensity the greater the attempted speed of change.
Superb protection against high-velocity impacts.
Mediocre protection against melee-speed weapons.
Movement impediment, possibly "tunable" to a small degree, letting you find your preferred balance of low-velocity protection and freedom of movement.

The "indestructability" part could come in a related flavor, letting the material be workable at low speeds, but otherwise resisting with a force roughly proportional to the mass of the piece (thus affecting design, and maybe still leaving steel superior for small pieces, justifying vulnerabilities and similarities to our world).

It would, at best, make a functional mace, though the property trade-off may still make it less practical than many alternatives. Not impossible, just an inferior idea.

As projectiles, I suppose they could be used in a long rifle if you held it steady long enough, though maybe not much better than a sling.

What if you wanted to use them as tiny bullets packing a huge punch?
I suppose small mass, air drag, and a need to put together a larger piece of the material to make use of the properties, could take care of that. Or low temperature tolerance. This could make for a separate SE question.

Rods from God, though? Goodbye, continents.

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Another answer could be that the material becomes exponentially stronger in relation to the size of a given piece of it. In the same way a large mass has more gravity, larger pieces of this material could be much stronger. Pieces small enough to be wielded, would be weak and break apart easily while a contiguous piece of it the size of a suit might be able to withstand an atomic blast. Once you get past a certain size (size of a suit or maybe smaller), It might get tougher, but it doesn't matter because at the size of a suit, it can withstand being tossed into the sun.

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There is a real life example of such a thing: Big safes or bank vaults. Yes, they could be destroyed, but it is impractical enough to do so that they are effectively considered indestructible. And the materials they are made of - concrete, hard steel - are not difficult to come by at all...

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A shaped charge, a HEAT round, or a long-rod penetrator will blow through the side of a bank vault like it wasn't even there; a HESH round will turn the contents of the vault into mincemeat without penetrating. Bank vaults are designed to stop bank robbers, not tanks. – Mark Jan 10 at 20:11
    
Well aware of that - but I think the example holds given it withstands the threats that it practically faces in the society it exists in :) – rackandboneman Jan 10 at 20:19
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@rackandboneman - I don't think the analogy holds up. If you use a safe or a vault as a "suit of armor", and lock yourself in, yeah, it's indestructible to things normally used to kill individuals...but it's not a suit of armor, it's your tomb. That's why nobody needs anti-tank weapons to kill humans. But if you say "No, I'll put it on treads or wheels and give it weapons," you're talking about a tank, and it's not indestructible to anything commonly used to kill tanks. Just sayin'. :) – user3573647 Jan 10 at 22:02
    
@user3573647 Actually, the analogy is poor because bank vaults do not need to protect the contents. Very few bank robbers are trying to destroy the contents of the vault. Opening a vault is easy, getting the stuff out is hard. In combat, the other side rarely will try to remove the armor off the opponent without harming them. – Aron Jan 11 at 4:56
    
@Aron - You lost me. I think he was comparing vaults and safes to armor because of their durability. And I'm pretty sure bank vaults DO need to protect the contents, not from being destroyed but from being stolen. And you say opening a vault is easy, getting the stuff or is hard. First off if it's easy to open, you need a new vault supplier. Second, if the vault is open, why would getting stuff out be hard? – user3573647 Jan 11 at 7:28

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