I found [this question](https://worldbuilding.stackexchange.com/q/118934/87100) on Worldbuilding Stack Exchange. A part of it interested me; specifically, the part about how, in Jay Dee's setting:

> 
> Guns are more efficient by use of liquid ammunition that solidifies
> when exposed to air (more ammo in a magazine)
> 
> 
> 

Presumably, this enables "more ammo in a magazine" by ensuring that most of the volume of the magazine is used to store ammunition.

This got me thinking about another, slightly different topic: liquid *propellants* that solidify upon exposure to air, as opposed to liquid *ammunition* in the general sense. In my version of this, the bullet - a solid projectile similar to a modern bullet minus its cartridge/case - is loaded, and then propellant is pumped in behind it and solidified. This may or may not involve separate magazines for bullets and propellants, but that's not relevant to the scope of this question. Instead, I'd like to ask: is the production of such a substance possible? If so, is it possible to do so on an industrial scale?

Such a substance would:

* be comparable to gunpowder stability-wise; not something that detonates on a whim like nitroglycerin, but not something that's impossible to ignite
* permanently and irreversibly solidify on contact with air, or some component of the atmosphere; yes, I recognize that this might cause some issues when applied to firearms; yes, that's the point
* take a liquid form prior to any form of contact with air/some component of the atmosphere
* have an energy density comparable to gunpowder

Assume a level of technology identical to that of the present day and an atmosphere identical to that of present-day Earth. Also, assume that minor heating, cooling, or pressurization devices are allowed as part of ammunition storage; I recognize that some substances are only a liquid under certain pressures and temperatures, and that those pressures and temperatures are not necessarily those on the outside of the magazine.

While this question is not tagged hard-science - for all I know, this is scientifically impossible, and, therefore, impossible to answer scientifically - I'll treat answers that use chemistry or physics to prove their point as better ones.

I have researched this, but the returns are *heavily* polluted by things such as [bulk loaded liquid propellants](https://en.wikipedia.org/wiki/Bulk_loaded_liquid_propellants) and [Gyrojet weapons](https://en.wikipedia.org/wiki/Gyrojet), which, while interesting, are not what I'm looking for.