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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.

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.

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!

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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!

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.

Here is a modern hunting air rifle.

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.

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"