I had the idea of a "void weapon", which would "generate cold" rather than heat on triggering. All the air from the chamber would be sucked out (thus "creating cold") and on firing, it would open the chamber: the difference of pressure would push the bullet out. However I'm not sure it would be powerful enough or even feasible.

I was wondering whether an endothermic weapon was possible. If so, how would it work ?

Conditions are:

  • Current technology
  • Must be endothermic: it can produce energy, but when shooting, the total sum of produced and removed energy must be negative. Energy stored in the bullet, if any, doesn't count: else that wouldn't make sense I guess.
  • The endothermic part of the gun is mandatory for it to work (don't stick an ice cube machine to an M16, or just put an icicle on a crossbow...).
  • Endothermic part can be part of a cooling system, but if you can come up with something a little bit more original, it'll be better (though it's not forbidden)
  • You can use the "void weapon" idea
  • Can be used and moved by just one person. Maximum size and weight: rocket launcher (with a backback)
  • Doesn't need to be reloadable or have more than one "shot" (was gonna say "bullet", but it can use something else, doesn't have to be a kinetic weapon).
  • Can kill one person without bulletproof vest
  • It's an experimental weapon: it doesn't have to be mass produced (almost infinite budget for R&D)
  • Bonus point if the endothermic part is visible (frost on canon, cold steam pouring out, etc.)

EDIT: after comments, removed hard-science: we just need approximate concepts.

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    $\begingroup$ NO! Creating a vacuum would require work to be done, which would create heat. ...the difference of pressure would push the bullet out. Again, no! If the front is open, air will enter through there; if it isn't, the bullet cannot escape. Second Law of Thermodynamics: you cannot recover energy from ambient heat $\endgroup$ – nzaman Dec 19 '18 at 15:44
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    $\begingroup$ @nzaman I think we’re okay, actually. There are spontaneous endothermic reactions that could feasibly used to create (weak) heat engines. Imagine taking a chemical cold pack and using that as the heat sink while using the ambient air as the heat source. Enthalpy-decreasing isn’t illegal as long as the entropy increase is suitably large. $\endgroup$ – Dubukay Dec 19 '18 at 15:53
  • $\begingroup$ @Dubukay: The OP states, Must be endothermic: it can produce energy, but when shooting, the total sum of produced and removed energy must be negative. Energy stored in the bullet, if any, doesn't count: else that wouldn't make sense I guess. That's completely different from enthalpy decreasing $\endgroup$ – nzaman Dec 19 '18 at 16:04
  • $\begingroup$ @nzaman If we consider the asker to be using “energy” synonymously with “a measure of the temperature of the system”, then we can certainly have an “energy” decrease as long as the system is destabilized in other ways. If you microwave a cold pack (don’t do this) it’ll get warm temporarily then burst, mix, and rapidly cool down. Thus the “energy” of the system has decreased overall due to the small enthalpy decrease and the large entropy increase. I agree that the system will always be in an overall lower energy state than before, but that doesn’t feel like the spirit of the question. $\endgroup$ – Dubukay Dec 19 '18 at 16:11
  • $\begingroup$ well evaporation is technically endothermic... $\endgroup$ – tox123 Dec 20 '18 at 4:04

Releasing highly pressurized gas normally results in cooling (there are exceptions, e.g. helium). The expansion results in energy loss which effectively cools the gas.

An example of this is literally called freeze spray (used to make animal waste more solid among other things). These typically use some form of fluorocarbon gas.

So if you could compress a gas enough and used the release of pressure to propel your projectile, you would end up with cold steam pouring out of the chamber after every shot.

Loss of pressure after each shot, may result in diminishing returns in terms of kinetic energy, unless each shot was backed with it's own pressurized container (which was said to be out of scope of the question).

Air powered rifles can be deadly. The Girandoni air rifle was used by the Austrian military as far back as 1780.

So with the right gas under the right pressure, you should end up with an endothermic reaction as part of the firing process.

  • $\begingroup$ An ordinary air gun ought to qualify here, except for the frost. $\endgroup$ – Zeiss Ikon Dec 19 '18 at 16:08
  • $\begingroup$ The idea behind the void weapon was to use gas expension to generate cold (just for the sake of having a gun that is endothermic actually) ... This solution is a lot more obvious, easier, and actually exists ! Gonna wait a little to see other answer, but this what I was looking for. $\endgroup$ – Asoub Dec 19 '18 at 16:27
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    $\begingroup$ Sorry, @TracyCramer, there's no way to get the gas out of the CO2 bulb fast enough to get excessive velocity from it. Even pre-charged air guns (at 3-4 times the pressure) are limited mainly by how much air you can route behind the projectile fast enough to contribute to velocity; almost all give subsonic muzzle velocity. Still, a 550 grain, .50 inch bullet at 700 feet per second is plenty for most game. $\endgroup$ – Zeiss Ikon Dec 19 '18 at 17:29
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    $\begingroup$ @TracyCramer Correct, it is -- but you won't get that from a bulb-fed CO2 gun with a heavy enough projectile to do that job. That figure comes from a 3000 psi (200 atm) pre-charge air gun, and is top-end for them. OTOH, the Girandone referenced above shot a .46 inch ball weighing 210 grains at 500 ft/s, and was plenty lethal. $\endgroup$ – Zeiss Ikon Dec 19 '18 at 18:37
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    $\begingroup$ Making a high pressure tank is not that hard, filling it is. 3000psi is the high end for single stage compressors, but industrial multi-stage compressors & tanks can reach 7000psi which would get you the same muzzle velocity as a standard handgun (and be very endothermic.) $\endgroup$ – Nosajimiki Dec 19 '18 at 21:21

The problem is that the most pressure this could produce is atmospheric pressure.

Let us imagine your endothermic BB gun. You have an awesome (here unspecified) endothermic reaction that turns the air in your vacuum chamber into liquid. Let us assume that after the reaction this cold chamber contains a perfect vacuum so external atmospheric pressure squeezes it down from the outside against the spring.. When you pull the trigger and open the chamber, ambient air at atmospheric pressure will rush in to the vacuum chamber. No longer squeezed down externally, the spring will uncoil, snapping the chamber back to full size and propelling the BB out.

Atmospheric pressure is 15 psi. A BB gun running on a CO2 cartridge has 900 psi. You will fire your BB. It will not go far.

Ralphie - be careful with the endothermic BB gun. You could still put your eye out!


thinking further, maybe you could have a very large vacuum chamber and gears like a bicycle. You would gear up the work done by that volume of atmosphere rushing in and moving a large piston to move a very small piston forward much faster. This becomes a single stroke vacuum engine which is legitimate. The gears would be on the outside. Don't get your cuff caught in them!

  • $\begingroup$ Well, for the void weapon, I though about a reaction (also unspecified, and also awsome) that would solidify the gaz ! What if the reaction continues with the bullet and prevents air friction by solidifying/liquidifying out of the way ? I don't think that makes a lot of sense, but that would prevent air friction. I like the idea of the vaccum engine too ! $\endgroup$ – Asoub Dec 19 '18 at 16:35
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    $\begingroup$ I have seen propositions for that vacuum launch tube thing for larger stuff - like a many km railgun bobsled tunnel to space where you want to accumulate a lot of kinetic energy without pesky gas soaking it up. halfbakery.com/idea/… $\endgroup$ – Willk Dec 19 '18 at 16:48

Already exists, its called an Air Rifle.

Really any compressed gas gun works. When fired the decompressing gas drastically lowers the temprature. Filling the reservoir is very exothermic that gets dissipated quite quickly until it reaches ambient temperature, it may not even be done in the gun itself, meaning when the weapon is actually fired it ends up very cold. Hunting air rifles exist so lethality is not an issue. Even better the technology has existed for almost a hundred years.

The rapid drop in temprature even gives you a nice smoke effect from condensation.
enter image description here

Here is a modern hunting air rifle.

enter image description here

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    $\begingroup$ The Austrian army used air rifles to good effect in the 1790s. Their enemies tried to get the Pope to ban them (as had been done with crossbows 400 years earlier) as "unfair" due to their lack of location-betraying smoke. $\endgroup$ – Zeiss Ikon Dec 20 '18 at 12:06

I'm going to go with a bi-metallic crossbow. The stock of the crossbow holds a vacuum canister full of Liquid Nitrogen with a valve that will inject the liquid into several small tubes in the bow arms when a secondary trigger is pulled.

To load, while warm, the crossbow has the bowstring pulled back and a bolt loaded as normal. When ready to fire, the secondary trigger is pulled flooding the bow arms with LN2 drastically reducing their temperature and due to interactions between the two metals drastically increase the strain as the bow arms try and bend forward increasing the throw strength of the crossbow.

After firing, small fins on the bow arms help them to warm back up to reduce the pull weight, assisting with reloading, while a vent dumps the expanding N2 gases from the arms.

  • $\begingroup$ So you're adding cold to a crosbow to make it more powerfull ? that's pretty smart and really original ! $\endgroup$ – Asoub Dec 20 '18 at 8:31

Since you took hard-science off, I'll throw two options out there.

  1. A super efficient micro-machine heat pump, extracts heat from the ambient air and pumps it into a gas chamber, which is used to propell the weapon's projectile. As it runs the heat exchanger on the outside could become icey. Ice reduces the efficiency so part of using the weapon is occasionly deicing the coils.

  2. A rail gun with superconducting magnets may need an elaborate cooling system to keep the superconducting coils below the critical temperature. This cooling system will include a heat exchanger, that will be hot, and superconducting magnets that will be cold. Invent a reason to have poor insulation on your magnets and the resulting less efficient system will have frost/ice on the outside.


I may be overlooking something in the restrictions, since this answer hasn't already been given, but why not just combine a (high tech, insulated and over-engineered) water pistol with a tank of liquid nitrogen? (sort of an anti-flamethrower)

Liquefied gas tends to be kept under high pressure anyway - a small quanta of Liquid Nitrogen is diverted to the Firing chamber, and additional pressure is applied so that when the front aperture opens the 'shot' is propelled out, much like an Archerfish, and freezes the target

The chamber is then flushed and reset for the next round - any excess Liquid Nitrogen evaporating would cause the barrel to steam.

For bonus fun, replace the Liquid Nitrogen with Liquid Oxygen and add a pulsed IR laser to ignite the oxygen-rich target - first you put them on ice, and then you set them on fire. We'll call it "Freezer Burn"

  • $\begingroup$ Somehow, I didn't even think about some nice "ice-thrower", this is great, and the Freezer Burn is even better ! Do you think liquid oxygen would burn or rather explode ? Also, I've done a little research, and flamewthrower also uses liquid (commercial one mostly uses gaz), this tends to "stick" everywhere. I wonder if there is some kind of endothermic viscous liquid that would do the same (liquid nitrogen as you said ? chemical cold pack content ? liquid oxygen mixed with something ?) $\endgroup$ – Asoub Dec 20 '18 at 10:18
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    $\begingroup$ If you want a "sticky" cold substance, then it needs to be something like a chemical cold pack that keeps actively cooling - with LOx or liquid Nitrogen the aim is for it to evaporate quickly and deal all the Cold damage at once for maximum effectiveness. On the "burn vs explode?" - that will depend on the material you are igniting, and how fast the fire would propogate. Diamond would burn, cotton would explode $\endgroup$ – Chronocidal Dec 20 '18 at 11:05

ok lets have a chamber of gas, with the bullet at the end (opposite to conventional guns). Endohandwaving instantly freezes the gas in the the chamber to a liquid, pulling the bullet back into the chamber. This is the gun "cocked" and leaving it cocked for long will start to show ice/frost forming on the gun. On trigger, endohandwaving stops, and the liquid rushes back into gas form at high speed, pushing the bullet out.

It's kind of just a fancy air rifle, but compressing the gas into cool liquid within the chamber, instead of having a separate canister of compressed gas.


Dean Ing, in Wild Country, wrote about a "chiller". It was a short-range gun that, among other things, included a "cold gas" ampoule in each cartridge, which was used (by mixing in the suppressor) to quench the powder gas from the otherwise conventional cartridge (and also to lock the gun onto the trigger finger of an unauthorized user).

In practice, the cartridge that could do this would have to use a refrigerant (R-134, for instance, though that has too low pressure), have a cold gas ampoule larger than a conventional cartridge, and carry the gas at extreme pressure (to compete with the pressure at the muzzle during the shot). For hand-waving cool factor, however, it worked fairly well.

  • $\begingroup$ So it actually prevents the gun from shooting and freezes the unauthorized user's hand on the gun ? $\endgroup$ – Asoub Dec 19 '18 at 16:41
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    $\begingroup$ No, it (via fingerprint sensor, in the novel) prevents firing, and uses the gas to drive a piston to physically capture the unauthorized user's finger between trigger and trigger guard. As I recall, it would usually break the finger in the process. $\endgroup$ – Zeiss Ikon Dec 19 '18 at 16:47
  • $\begingroup$ @ZeissIkon yikes, better hope your kid doesn't get into the gun cabinet. Unless it doesn't do that when the gun is empty, they've just made unloaded guns needlessly dangerous. $\endgroup$ – Martin Carney Dec 19 '18 at 22:08
  • $\begingroup$ The chiller was (in the novel) issued only to agents of a government agency, the trajectory of which was much of the plot of the novel, and formation of which ended up the previous book, Systemic Shock. $\endgroup$ – Zeiss Ikon Dec 20 '18 at 12:02
  • $\begingroup$ But since the gas was contained in the cartridge with the powder and bullet, no, it couldn't do that when unloaded. $\endgroup$ – Zeiss Ikon Dec 20 '18 at 12:03

Let's first start with the vacuum part.

The only way to use vacuum to propel a projectile is to generate the vacuum in front of the projectile and let atmospheric pressure do the pushing, accelerating it. More or less what we do when we use a straw to drink from a glass.

The problem with this approach is that the pressure gap between vacuum and atmosphere is a flimsy 1 bar, which compared to the instantaneous pressure generated by conventional projectile is really low.

On top of that you would need a really precise mechanism to seal the barrel and open it right when the projectile is next to its end, else the pressure wave of the entering air would reduce its velocity.

You already see that this approach is inefficient and complicated, but in principle possible.

  • $\begingroup$ As you putted it out, I don't think 1bar would be enough to make it a gun. Even without the air somehow entering (your barrel seal, or some kind of air typhon on the entrance of the barrel ?), would the size of the barrel (vaccum chamber) change the gun's power ? I guess the "void acceleration" would be limited at some point too. $\endgroup$ – Asoub Dec 19 '18 at 16:38

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