So, imagine that we're using technology that allows us to fire superheated bullets, and we decide to implement it against three types of opponents:

  • Unarmored, wearing just normal clothes
  • Lightly armored, the armor just lessens the impact of a normal bullet so that normal grazing shots are ineffective, and lessens the damage against direct hits
  • Heavily armored, the armor protects even against some direct hits

Say, we use non-lead bullets, and they're heated to the point when they're at 1000 degrees Celsius at the muzzle exit, with the metal that we use being similar to a non-heated lead in this heated state. What kind of damage we're looking at? And what if the number is 2000 Celsius instead? 3000 Celsius with Tungsten bullets?

Additional question: what countermeasures would be implemented against this kind of weaponry?

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    Are we to assume that the muzzle is able to withstand that intense heat without melting, warping, ablating, or otherwise sustaining damage? – F1Krazy May 15 at 9:32
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    Yes, we're assume that the weapon is able to withstand at least one consecutive shot – Baka-Mastermind May 15 at 9:34
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    I think the first question to ask here is "what is the normal temperature of a rifle bullet hitting a target", from a quick read, they're already quite hot. – Separatrix May 15 at 9:49
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    A NATO 5.56mm round weighs about 4g. If you make the round from a material that can hold a lot of heat (aluminum), one round at 1000 C is about 3600J. If that round lodges in a 150lb-mostly-water target, it will raise the temperature of the entire system by a negligible 0.13 C...though it may tend to cauterize the entry wound. The shock waves traveling through the body cause massive trauma, but that's not related to the temperature of the round. – user535733 May 15 at 12:59
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    In what sense are these projectiles "superheated"? "Superheat" typically refers to heating a fluid above its boiling point - for example, superheated steam as used in shipboard propulsion and power generation plants. – Bob Jarvis May 15 at 16:08

14 Answers 14

up vote 63 down vote accepted

The name of the game for weapons is the amount of energy they transfer to the target and how fast and how concentrated it is when it gets there.

The kinetic energy of a fast-moving projectile transfers energy very effectively: It slows down a lot (leaving the kinetic energy behind to damage tissues).

The heat energy of a hot projectile? Not so much. It has only the milliseconds it's traversing the body to transfer heat and not much is going to be transferred. (And besides, if you've already made a hole through a person, how much more damage do you need to do to take him out of the fight?) As other answers have noted, the heating decreases the strength of the projectile which may decrease its effectiveness. On the negative side, again, as pointed out in other answers, the hot projectile adds substantially to the complexity of the gun that fires it and to the logistical difficulties.

Perhaps worst: The gimmickry needed to make the projectile hot will almost certainly diminish its speed, quite possibly making it less effective overall.

Protection against? Not much more, really, than already needed to stop ordinary bullets. (I suppose you'd want to avoid using flammable armor.)

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    Upvoted for being concise and avoiding excessive flippancy without delving into masturbatory levels of technicality. I'd add (as has been mentioned by others) that a softened bullet would actually lose at least some of its armour-piercing qualities and become less effective. – Ruadhan2300 May 15 at 13:01
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    @Ruadhan2300 What's that you say? Delve into masturbatory levels of technicality? OK! ;) – Schwern May 15 at 19:39
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    Your arguments break down if the bullet doesn't leave the body. – vsz May 15 at 20:48
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    Once you get past the armour, I'd expect a good reference for what to expect is the "red hot nickel ball" series of videos on youtube. Particularly the one with the watermelon. Net result, it burrows a bit, boils fluids around itself for a time and generally inflicts local thermal damage. Very nasty and extremely painful, high likelihood of thermal-shock, infection and death. Basically a guaranteed warcrime. There are specific clauses (protocol 3 if you're curious) in the Geneva convention about the use of incendiary weaponry on humans too. – Ruadhan2300 May 16 at 11:27
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    @Sudix you are making me think of HEAT (shaped charge) projectiles. Bullet sized ones won't be very effective though. – Baldrickk May 16 at 13:14

What Damage Does A Normal Bullet Do?

As with any new weapon, we first have to ask whether this is a marked improvement over what we already have. Is it worth the bother?

Let's run the energies involved, because bullets wound by transferring energy to the target. Let's assume a standard M855 5.56 NATO rifle bullet fired from an M16 with a 20" barrel, because that's what you'll see on a battlefield, and anything smaller is almost useless.

Leaving the muzzle, the bullet has 1900 J of energy. This drops off to about 1300 J at 100 meters, and about 600 J at 300 meters. After that, you probably won't hit anything.

This is already enough energy to pierce a steel helmet and Type 1 and 2 body armor. Upon hitting flesh, the kinetic energy of the bullet, plus cavitation effects, does some very bad things indeed.

Ow.

Source. Also, ow.

This is already enough to give the target a pretty bad day. Can heating the bullet significantly improve on that?

First, Some Problems

There's the problem of how we heat just the bullet to such high temperatures quickly without also heating the surrounding metal of the rifle. There's also the problem of heating, and setting off, the powder charge. Tiny lasers inside the chamber? What's the power source? This all sounds very fragile and expensive.

Second, lead melts at 327°C. Steel at 1500°C. Let's go with tungsten at 3422°C for our bullets. Very expensive tungsten.

Third, a high temperature bullet would severely damage the rifling on your rifle no matter what it's made of. You could only fire a few of these before you'd be unable to consistently hit targets and have to change barrels (and maybe other parts).

Fourth, a high temperature bullet might melt your rifle. Melting points vary by alloys, but generally aluminum melts between 500°C and 1300°C. Steel at about 1500°C. And you certainly can't use any composites. You'd need a very expensive tungsten chamber, barrel, bolt, and receiver.

IANAL, but legally you can probably shoot as many soldiers with this as you like so long as they're not near civilians. If you used it in a city you'd probably be in violation of the UN Protocol on Prohibitions or Restrictions on the use of Incendiary Weapons Article 2 Section 3 which nearly everyone has signed.

It is further prohibited to make any military objective located within a concentration of civilians the object of attack by means of incendiary weapons other than air-delivered incendiary weapons, except when such military objective is clearly separated from the concentration of civilians and all feasible precautions are taken with a view to limiting the incendiary effects to the military objective and to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects.

We'll let the engineers solve those problems, and the accounts figure out how to pay for it all, and the lawyers to make it legal.

How Much Energy Is In A 1000°C Bullet?

If we heated the 4 gram tungsten bullet to 1000°C what sort of energy would that transfer to the bullet (and later to the victim's flesh)?

$$ \text{energy} = \text{specific heat of tungsten} \times \text{mass of bullet} \times \text{temperature change} $$

$$ \text{specific heat of tungsten} = 0.132\,\frac{\mathrm{J}}{\mathrm{g} \times \mathrm{K}} $$ $$ \text{mass of bullet} = 4\,\mathrm{g} $$ $$ \text{temperature change} = 1000\,\mathrm{K} $$

528 J. Not an insubstantial amount of energy, but significantly less than the bullet itself delivers. And it's also going to lose that energy, radiate it away to the air, as it travels to the target. Tungsten has a very low specific heat which means it does not retain heat well.

Let's get this out of the way, this will add nothing to its penetration capability. It won't "burn through" the armor, there just isn't enough energy. To give you an idea, 528 J is about the energy of a decent photography flash being relatively slowly and inefficiently transferred to the air and armor. Whereas a bullet transfers all of its kinetic energy very quickly by slamming into the target and rapidly decelerating.

"Ah ha, but there will be this burning hot hunk of metal inside the flesh of the target!" Well, this brings us to our next problem: water. Water absorbs heat like nothing else.

Our 4 grams of hot bullet will be cooled as it passed through the surrounding flesh. Flesh is mostly water. Unlike tungsten, water has a very high specific heat and can absorb over 30 times as much energy as tungsten by mass. And it takes even more energy to turn liquid water to steam (which would be really bad inside a body).

How much would 528 J of heat to do, say, just 10 grams of water representing the flesh in contact with the bullet?

$$ \text{temperature change} = \frac{\text{energy}}{\text{specific heat of water} \times \text{mass of water}} $$

$$ \text{specific heat of water} = 4.18\,\frac{\mathrm{J}}{\mathrm{g} \times \mathrm{K}} $$ $$ \text{mass of water} = 10\,\mathrm{g} $$ $$ \text{energy} = 528\,\mathrm{J} $$

13°C change. Lukewarm. Likely no appreciable additional effect to an already heavily traumatized area.

What about 2000°C? Double the temperate change, double the energy: 1056 J which can heat 10 g of water by 26°C.

What about 3000°C? Triple the temperate change, triple the energy: 1,584 J which can heat 10 g of water by 39°C. If it was already 37°C (body temperature) you're at 76°C which is hot enough to scald, but it's just 10 grams.

You might have even done them a favor by cauterizing the wound.

Modern Incendiary Rounds

Incendiary rounds exist primarily for two reasons: tracer and anti-material. Rather than heating the bullet, the incendiary component is provided by a chemical reaction. This is much more energetic than just heating metal.

Tracer rounds are designed to glow so you can see them in flight to assist with aiming. It's more about light than heat.

Anti-material rounds are designed to use their kinetic energy to penetrate the hard outer shell of a vehicle or building, and once through ignite. Usually the bullet contains some chemical that spontaneously combusts on contact with air after the bullet deforms on hitting a hard target. Unlike the wet flesh inside of a person, a combat vehicle contains lots of dry, highly flammable things inside to ignite and burn. Fire is a combat vehicle crew's worst nightmare.

However, your typical intermediate combat round (5.56 NATO or 5.45 Soviet) is too small to carry a significant incendiary charge. You need to scale up to full power rifle rounds like 7.62mm for the bullet volume to be large enough to make an incendiary payload worthwhile.

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    How about 10,000 degrees Celsius? Melts the rifle: all you need is a mold with you!. – Redwolf Programs May 16 at 2:42
  • @RedwolfPrograms impractical ... however you could propel and guide the bullet it with magnets while firing aka rail gun firing metal plasma. – CaffeineAddiction May 16 at 22:13

Tracer Round

What you really want exists, it's called a tracer round.

A tracer round works like a bullet sized flare. The tracer projectile is filled with a pyrotechnic flare material, made of a mixture of a very finely ground metallic fuel, oxidizer, and a small amount of organic fuel. It burns at several thousand degrees. If fired into dry brush or grass, it will start a fire.

There are three types of tracers: bright tracer, subdued tracer and dim tracer. The dim variety will give you the heat you need. They come in armor piercing varieties, but honestly if armor piercing is your thing, there's better projectiles.

Good luck with your hot buwwets.

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    A tracer round works like a bullet sized flare. The tracer projectile is filled with a pyrotechnic flare material, made of a mixture of a very finely ground metallic fuel, oxidizer, and a small amount of organic fuel. It burns at several thousand degrees. If fired into dry brush or grass, it will start a fire That... Is just... Beautiful! – Renan May 15 at 19:32
  • So what happens when you fire a tracer round at a person? – MissMonicaE May 15 at 20:16
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    @MissMonicaE The same thing that happens when you fire a similar buwwet at a person. It's a real bullet with real inertia that can cause real damage and severe burns. – gwally May 15 at 21:17
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    Tracer rounds don't cause fires reliably, since they aren't designed for that. They sometimes can, but most often don't, even when striking flammable material. They are only designed to be seen, the heat is a side-effect. If you want a bullet specifically designed for starting fires, check out incendiary rounds: en.wikipedia.org/wiki/Incendiary_ammunition – Jared K May 15 at 21:50
  • @JaredK Based on personal experience, I would have to strongly disagree. It doesn't light every time, but when it does it's hot enough to start a raging fire. The point I was making is that at several thousand degrees Fahrenheit, it meets the criteria for a hot buwwet. – gwally May 15 at 22:16

The higher the temperature of the metal, the more easy it is to deform it.

So you can end in a situation where your bullet doesn't pierce the target, but rather splash on it if the impact surface is sturdy enough.

You have basically achieved the same result of an arresting rubber bullet with a more complicated and expensive implementation. A normal armored surface would be sufficient to counter it.

Though I doubt that the temperature will stay that high after leaving the muzzle: air flow around the bullet during the flight would surely cool it down sensibly.

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    Yes, I understand that, hence I specified that we don't use lead with its melting point being slightly above 300 degrees. The question specified that used metal is of the same "softness" under these conditions as a normal lead bullet. – Baka-Mastermind May 15 at 10:17
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    However, Dutch is right. No matter what material you use, unless you're in the vacuum of space the bullet would cool really, really quickly due to the rush of air at 1,000+ FPS. Even if you assume a hard bullet, the kinetic impact is thousands of times more relevant than the heat, which would only burn a bit and would never ignite anything or melt anything (remember how fast that sucker's moving, there's no time for thermal transfer). – JBH May 15 at 14:26
  • @Baka-Mastermind A hand-wavy "magic" bullet that generates it's own heat constantly would be more ideal if you wanted the bullet to maintain its heat. – Schrodinger'sStat May 15 at 15:40
  • @Schrodinger'sStat, just use plutonium to make the bullet. – L.Dutch May 15 at 16:24
  • @Schrodinger'sStat Not nice storing lots of ammo actually build up heat, even if you do use not heat triggered explosives the arsenal can be too hot the poor logistics officials – jean May 15 at 17:07

Short answer: Not much damage. Also massive damage.

Long answer:

Effects on organic targets

Bullets are designed to poke a hole in you and hopefully pass through something important, like a nerve or artery. Fancy features like fragmentation, yawing, hydrostatic shock or hollow-point expansion are all in the furtherance of this goal, maximizing the size or number of the holes they poke in you.

Thing is, bullets move really fast [citation needed], so the time spent hole-poking is minimal. Anything fast enough to be an effective weapon at range is going to zip straight(ish) through you, giving it very little time to transfer heat to its target. If, by luck, the round managed to lodge in the targets body the effects might be grisly, but this will be the exception, not the rule.

In summary, not really an improvement over normal bullets.

Effects on your Diplomatic/Public relations

So, you've created a weapon that isn't any more powerful or useful than a normal rifle, but can - in some cases - inflict truly horrifying and needlessly cruel burning injuries to enemy combatants. Best case scenario is that you're painted as an absolute monster by opposition media. Worst case scenario you go to the Hague for war-crimes trials.

To drive home the point, read this article about AR15s, and consider that 5.56x45 NATO isn't designed to do anything particularly nasty, and isn't very powerful. Now, in your minds eye, imagine the news talking about your melta-bullets immolating women and children.

In summary, irreparable damage to your public image.

Listen kid, you wanna set'em zombies1 on fire, you gotta do it right. Don't bother preheating your ammo, use proper incendiary ordnance.

There are these shells, called Dragon's Breath:

Jingle bells, shotgun shells

And this is what firing them from a regular double click looks like:

The roof! The roof! The roof is on fire!

It doesn't matter how much armor someone is using. Full riot gear will have the fire coming at your neck. Only full asbestos armor would protect against these.

Range is quite short, though. Don't expect to win a field battle with these.

1 I hope it's zombies you are preparing yourself for.

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    Set zombies on fire, all you get are burning zombies. They don't panic, don't feel pain, don't suffocate from smoke. But they will set your defenses on fire while trying to climb them. – SPavel May 15 at 12:49
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    If you need to set zombies on fire, go for a Russian Thermobaric rocket launcher. The heat impulse will vapourize any zombies in the impact zone, and the effect will be several hundred metres from your position, which is very important when dealing with a flame weapon. – Thucydides May 15 at 15:20
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    If you need to set zombies on fire, nuke the entire site from orbit. – Jonas Wielicki May 17 at 11:42

Sounds like it will only be useful on unarmoured targets (on which normal bullets work just fine).

The impact on armour is so fast that hardly any heat transfer happens before the bullet comes to a stop. So unless there is enough penetration for the bullet to stick and transfer heat after impact, the added temperature is negligible.

Shooting at unarmoured victims will probably deal some more damage, because you'll cook part of the target after it is hit, creating a larger zone of damaged tissue.

  • The question states that we use metal that in this heated state is similar to a normally fired lead, so the efficiency against armor should be about the same. – Baka-Mastermind May 15 at 9:38
  • @Baka-Mastermind, Even bullets made from Tungsten would struggle to perform this action. the only thing that could do this would probably be Uru, the mythical metal that Thor's Hammer is made from. – Blade Wraith May 15 at 11:58
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    You do realize that, if this is true, then it is basically just as effective as a blaster. Because blasters don't use metal bullets at high temperature, and instead use tibanna gas that is in a plasma form to do damage, blasters are more efficient. Waste of metal alloys is what I think of bullets when you can have a good blaster instead. – Darth Vader May 15 at 12:39
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    I was questioning why blasters were being discussed right up until I saw the name..Question retracted :P Get outta here sithlord! – Ruadhan2300 May 15 at 12:57
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    Wouldn't a heated bullet cauterize the wound, preventing internal bleeding? Maybe that's a little too optimistic. – pipe May 15 at 13:25

This seams like a stepping stone to what is commonly refered to in sci fi games as plasma weapons, firing a (somehow self contained) blob of super heated plasma. There's actually a few good youtube videos on how these might work.

The problem with these sorts of weapons is they are severely limited in their usefulness. but i'll answer your question the best i can first

Damage

While a regular bullet can easily inflict severe damage and death on hitting it's target, a glancing blow is often not fatal, unless left untreated, the victim will bleed to death or the wound will get infected. A super heated round is as likely to kill on impact, but it will also cauterize the flesh on passage through as well, which makes bleeds and infection slightly less likely, so it could i stress could be actually less lethal in certain circumstances against unarmoured targets

and Modern Body Armour can take the heat of a bullet impact, so although the heat is higher, simple modern ceramic plates would shrug this off quite easily, the only advantage is the risk it could ignite the material hold the ceramic plates together, but this is usually Kevlar, which is actually not bad at dealing with heat and fire, not great but not bad. and this known is only because current Tracer rounds already risk setting things on fire when they hit flammable materials

As L.Dutch explained, the hotter the material the more likely it is to deform, this means there will be no Armour piercing round for the weapon even tungsten would not be able to stay solid enough at those heats to pierce armour. and tungsten has the highest melting point in nature.

However there are several other issues to consider with this weapon system

Accuracy

Bullets get their accuracy by the rifling of the barrel, if the round was super heated (and as above more likely to deform), it would not be able to gain this rifling as easily and therefore you have a much less accurate weapon, unless you made the barrel impractically long, longer barrel more weight to carry and more barrel to be cleaned.

Price

Modern conventional firearms are relatively cheap to produce, they need decent steal to withstand the heat from the weapons firing and can go through hundreds of rounds before melting the barrel, a weapon that had to fire 1000 degree on impact rounds would probably need to fire them at about 2000-3000 degrees to accommodate the loos of heat energy over the distance of the shot, and somehow that barrel and breach not become damaged by this heat, this is something even Tungsten would struggle with and tungsten is a lot more expensive then Steal. then theres the price of the ammunition...

Speed

A soldier can fire off hundreds of rounds a minute if he can load the rounds quick enough, if the Super heated shots need to become hot as they are fired then the rate of fire would have to be far slower to make sure its safe to fire the next round without cooking off in the breach due to the heat of the previous round fired.

Storage

If the ammunition was stored "cold" then it would have to be able to gain that massive heat quickly on firing, meaning a very high energy potential, similar at least to high grade explosives. so storing it would need to be similar to those same high grade explosives, if they were stored "hot", then it breaks the laws of thermodynamics to allow this heat not to dissipate over time, so the shelf life of the ammo would be very short

The need for sunglasses

Something most don't consider is the intensity of light given off by materials that are in the 2000-3000 degree range if a solder fired this weapon as the round left the gun he would be left at least temporarily blinded by that light, meaning he'd be unable to see in order to take the next shot...

Basically much like the famous Plasma weapons in most futuristic FPS games it is just not a practical weapons system.

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    No no noooooooo cauterization is a CAREFUL application of heat over time, not a sudden heatburst. For a medical procedure on myself they used an electrical cutter, after the cuts they sealed it with the same cutter by using a fraction of the energy and applying it over a time. – Demigan May 15 at 13:35
  • Demigan makes a vitally important point. What you'll get from this is a partial cauterisation at best around the area where your bullet comes to rest, and internal bleeding, thermal shock, infection and death for the rest of it. On top of the usual impact-trauma of being hit by a bullet which on its own is often enough to kill. – Ruadhan2300 May 16 at 11:37
  • I did not explain myself very well, the cauterization concept was more based on rough figures with Plasma based weapons, where the plasma itself would leave hot material behind in the wound, which in turn would have the potential to cauterize the wound. as we do not know what material the bullet would consist of this would still be a possibility. i will admit that complete cauterization is very very unlikely, but it does not require "Careful" application of heat. it has been documented many times using hot Irons etc by very untrained individuals – Blade Wraith May 16 at 12:54

What exists today that satisfies the effect you're looking for:

While this doesn't quite answer your question, there are bullets that do both of the things you want and are also fairly exotic: Incendiary Depleted Uranium bullets

Depleted Uranium is a more dense material than what it is being fired at (in part because it is very rare with respect to Steel.) This helps it penetrate most targets very easily. Because of this penetration, it happens to then be quite effective as a carrier of Incendiary munitions* because it will deliver them to the inside of the armored thing.

*Note: might be the wrong word here.

If this is scaled down it can still be useful against unarmored targets provided the delay for the explosive is such that it would generally travel a couple inches before exploding.

Use in other fiction works

It's worth noting that (while it's all handwavium) this is what is used in Warhammer 40k essentially; and they call their Bolts "Explosive Penetrators" IIRC.

What is wrong with the concept and how to fix it

If you're looking for super-heating the target to soften the armor, the reason it works is because the armor is being superheated, not because the ammunition is. This tends to occur with Anti-Tank munitions; where part of the munition is designed to super-heat the tank's armor prior to the impact of the the rest of the munition, allowing it to penetrate much more easily (to the point that modern munitions on a successful hit are normally one-hit-knock-outs; "successful hit" not counting reactive armor and other technological countermeasures to this style of munition.)

In your case you could end up with a weapon that fires two different munitions in rapid succession (on the assumption you need super-heating to be part of the process) one that attempts to melt the opponent's armor via some reaction or application of thermite or something; and a second that hits the target a moment later and more easily pierces it.

For soft targets the first round would likely be deadly since its going to burn them severely AND cause kinetic damage, aside from whatever the effects of the second round are. For harder targets you will damage the armor on one side and release a fragmenting or bouncing projectile into their suit, almost assuredly causing terrible damage. In general, all targets that require penetration will have a fragmenting or bouncing bullet problem, and all others will get hit twice and have extreme burns on top of two applications of severe kinetic damage.

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    "one that attempts to melt the opponent's armor via some reaction or application of thermite or something; and a second that hits the target a moment later and more easily pierces it" - actually I would attempt to get something very hot inside an opponents armour, not melt the armour to get a hole. Heated sand was used at the Siege of Tyre (332 BC) to great effect. – BentNielsen May 15 at 20:54
  • @BentNielsen Interestingly, I had forgotten about Depleted Uranium's other attributes; one of which it is apparently quite flammable at high temperatures, so when it punches through hull the entire round is actually causing high heat as well. (this is to say that it seems the military agrees with you haha) – blurry May 15 at 23:07

Besides the other good answers, there is something called the high-explosive anti-tank (HEAT) warhead. This isn't a standard rifle round, rather something that is usually rocket style fired, like in an AT4 (https://en.wikipedia.org/wiki/AT4) setup.

What the HEAT round does is get fired at a low velocity, is sometimes rocket propelled, and, on impact, explodes a shaped charge to superheat metal to penetrate tank armor.

https://en.wikipedia.org/wiki/High-explosive_anti-tank_warhead

The warhead functions by having the explosive charge collapse a metal liner inside the tank to form a high-velocity superplastic jet of liquid metal. This concentrated liquid metal jet is capable of penetrating armor steel to a depth of seven or more times the diameter of the charge (charge diameters, CD) but is usually used to immobilize or destroy tanks.

These were originally developed for WW2, so they've been around for a while, and have been effective against tanks up to the most modern version of 2010-era battle tanks, where they are only less effective rather than ineffective.

FYI, and also from the HEAT round Wiki page:

Contrary to a widespread misconception (possibly resulting from the acronym HEAT), the jet does not melt its way through armor, as its effect is purely kinetic in nature.

This doesn't seem like it could easily scale down to a rifle/pistol round, but it uses a heated metal to inflict damage to a target, with that damage being more effective than a "cold" slug.

Speaking in purely theoretical terms, in a future with bullets designed to inflict thermal damage upon an enemy the most realistic probability wouldn't be to fire a heated bullet at someone. It would be optimal if a technology was invented to intentionally pierce into the target before stopping it's own momentum and releasing a thermal payload, bursting into flames, or even becoming explosive (which would probably cause much more non-thermal damage than thermal damage). As far as damage inflicted against different armor types that's entirely dependent on the technology and intent of the round. A theoretically perfect round of the future would be able to pierce any armor equally while still able to stop itself in the center mass of the target this causing fatal injuries with every round that hit. Realistically there would probably be similar round types to what we have today. Using a round designed for armor would probably be unable to stop itself in the center mass of an unarmored target for releasing the payload but would still inflict kinetic damage. A regular round probably would have a lower chance of piercing armor but would still inflict kinetic damage from the impact.

The accepted answer is accurate assuming you're using projectiles not designed to expand on impact (in other words, you're either target shooting, or work for a military or paramilitary organization bound by the Geneva Convention), with a few caveats:

  • Armor would be more effective provided it's fireproof. Heating most materials makes them more malleable, which means they will deform more readily on impact. Assuming you're using, say, steel, heated to the temperatures in question, that's soft enough that pretty much anything a human can't dent with a punch will work as armor (again provided it's fireproof).
  • Hitting a bone inside a target would be significantly more lethal. Bone is likely hard enough that the bullet would deform against it instead of shattering it, unless the round is way more powerful than your typical 5.56x45mm NATO rounds, and possibly even more powerful than 7.62x51mm NATO rounds. This in turn would make the bullet behave like a hollow point or soft point, instead of a fully jacketed round, and thous cause larger wound cavity and stick around inside the target (see below for the impact that would have).
  • Glancing blows would be a lot more dangerous. Getting caught across the cheek with a regular 5.56x45mm NATO round will hurt like hell, but its not going to cause any significant damage beyond leaving a nasty cut and possibly a minor friction burn. Having the same thing happen with one of your superheated bullets (even the exact same size) is going to give you the cut, cauterize it (which is actually a good thing for you), and likely give you at least second degree burns (which is a very bad thing for you), and that's assuming it doesn't impact somewhere where your jawbone will provide resistance.

If instead you're using a bullet designed to either expand on impact, or in some other way not penetrate through the target, things change. In particular:

  • You have longer contact with the target. More heat would be transferred, and thus more total energy. This would in most cases probably cauterize the wound unless it's intersecting a major artery or vein, though at the same time it would likely cause some pretty severe internal damage (technically worse than third degree burns).
  • Because the projectile is softer, expansion would be more reliable (thus likely causing a larger wound), but it would not penetrate as deep (because it would start to expand sooner).

Note that this all ignores cooling of the projectile in transit (it's got airflow over it, so it will cool faster than some random bit of metal just sitting there) and the potentially extremely complicated internal ballistics of such a weapon (how do you keep the barrel from cooling the projectile too much, how do you keep the near molten metal from building up inside the barrel, what is the best option for rifling in terms of the now different ballistic characteristics of the bullet, etc).

keep in mind that any significant heat transfer takes a noticeable amount of time that increases based on the thickness and atomic properties of the materials. The amount of energy gained from superheating a bullet in trade for the decreased impact force from making the bullet more malleable is likely to be negligible, possibly negative given that the bullet will lose its heat rapidly if it's used in atmosphere

I do not think it would be better against unarmored or weakly armored opponents since heating the projectile would likely have less hardness. However this could be used as some sort of "hollow point" effect : less hardness means the projectile could split more easily and do more damage to soft targets. If this projectile splits, it could stay in the body instead of completely going through and the heat could actually do damage since it would have time to give energy to the target.

However, most of the damage would likely be ballistic and heat my actually prevent internal bleeding by cauterizing internal wounds.

As for heavier armour, if your projectile can "stick" to the armor after impact instead of fragments flying everywhere, this could melt a hole in armor and eventually destroy it.

Keep in mind that this is hypothetical and the effectiveness is not guaranteed. The gun needed to fire such projectiles would be much more complex and expensive than current guns and current guns can already bypass armor (sabot bulets/shells for example) or do more damage to soft targets (because hollow points already exist)

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