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Most of small arms used nowadays designed for rather specific boundaries of temperature, humidity, atmosphere and so on.

Let's consider a scenario where there is a human with an ability to sustain a very high and very low temperature of the skin surface (an X-man with such ability if you will). It is obvious that he would not be able to use a regular handgun when he is too overheated (the gunpowder within ammo detonates from heat) or too frozen (moving parts and gun grease get stuck together).

What would these critical temperatures be?

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    $\begingroup$ Seems like the sort of thing a Google search would clarify. (which I won't attempt, because I'm at work and like having a job) $\endgroup$ – AndreiROM Jun 17 '16 at 12:04
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    $\begingroup$ @AndreiROM I considered the same thing, but realized that, compared to the other things I've Googled for questions around here or my own research, this one is pretty innocuous. I linked one such result to Mike L's answer. $\endgroup$ – Frostfyre Jun 17 '16 at 12:19
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    $\begingroup$ I read that title so such a wrong way, I thought the OPer was on about a persons arms and was confused as to why the hand temperature would matter. This didnt click till I read handgun/gunpowder/ammo $\endgroup$ – Mr.Burns Jun 17 '16 at 12:20
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    $\begingroup$ @Mr.Burns - just read the title with that interpretation and I literally laughed out loud. Oh, priceless. Or maybe I'm just tired. Still, well done, sir. $\endgroup$ – AndreiROM Jun 17 '16 at 12:21
  • $\begingroup$ Rubber handles? Given proper insulation, maybe that's not really a problem. $\endgroup$ – AmiralPatate Jun 17 '16 at 12:29
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Varying by weapon. There are specialised weapons designed for warfare in extreme climates that can work in very low temperatures.

There is a variety of problems you might encounter with guns, and the first ones to appear are thermal contraction/dilation (a big problem in precision machinery like firearms), which is entirely dependent on the design and engineering. Linear thermal expansion coefficient of steel is about $10 - 20 \times 10^{-6} K^{-1}$, which means that a $~10\text{cm}$ rifle bolt will expand by $0.1\text{mm}$ when you raise the temperature by $100 K$, which may already be a problem in an automatic weapon.

Another problem that might appear is cold-short, which is when a steel suddenly becomes brittle at temperatures around 0°C, but again this is entirely dependent on the type of steel the thing is made of.

Finally, you could completely break the firearm just by heating or cooling it too fast; the various parts are quite tightly fitted, and if you change the temperature too quickly, some contract or expand before other which may cause enough mechanical stress in critical parts to break them and make the gun entirely non-functional.

Eyeballing it, if you want to be sure that a gun won't fire, cooling it below -100°C or heating it above 100°C will most likely do the trick.

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  • $\begingroup$ A quick search found a gun-hobbyist forum that mentioned ammunition autoigniting between 135 and 188° C. $\endgroup$ – Frostfyre Jun 17 '16 at 12:14
  • $\begingroup$ @Frostfyre Right, I left that out because I'm pretty sure a gun will stop working before then. $\endgroup$ – Mike L. Jun 17 '16 at 12:16
  • $\begingroup$ @MikeL. Either that, or an unprotected human arm will stop working before then. $\endgroup$ – a CVn Jun 17 '16 at 12:29
  • $\begingroup$ @MichaelKjörling I'm not asking about safety (it's quite obvious that this is far from any definition of safe), I'm asking about temperature. $\endgroup$ – Mr Scapegrace Jun 17 '16 at 12:35
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Weapons would need to be made of alloys that were largely invariable in size as a result of temperature change simply due to the amount of heating resulting from use of the weapon. While they can overheat in prolonged use, the autoignition temperature of your ammunition is going to be the more significant factor.

As an example of the heating from use, here's a guy firing an AK until it catches fire: https://www.youtube.com/watch?v=eNAohtjG14c

(Don't talk to the Germans about this, the G36 rifles lose all accuracy when they get hot.)

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