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I found this question 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 and Gyrojet weapons, which, while interesting, are not what I'm looking for.

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  • $\begingroup$ "take a liquid form when not in contact with air/some component of the atmosphere" Does it mean "revert to a liquid form once the contact with the atmosphere is interrupted" or just "as long as never in contact with the atmosphere, it stays liquid"? Would you mind to resolve the ambiguity, please? $\endgroup$ Nov 27 '21 at 8:31
  • $\begingroup$ @AdrianColomitchi "As long as never in contact with the atmosphere, it stays liquid"; working on it. $\endgroup$
    – KEY_ABRADE
    Nov 27 '21 at 8:33
  • $\begingroup$ You need the propellant to solidify for one instant, before you fire the gun and vaporize it? This seems like a tree-falls-in-the-forest situation... $\endgroup$ Nov 27 '21 at 15:10
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    $\begingroup$ permanently and irreversibly solidify on contact with air in that case, i guess every ammunitions manufacturer would just create small granules of the stuff via this phenomenon, and fill the bullets with those ganules. basically taking you back to solid explosive material. $\endgroup$ Nov 27 '21 at 16:02
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    $\begingroup$ You don't want a solidifying propellant because it would solidify into a single block without special mechanisms. Gunpowder is a powder for a reason: to provide a large combustion surface area for rapid deflagration. Moreover, the powder is specifically and uniformly sized to guarantee a specific combustion rate; too little and the projectile dribbles out the barrel, too much and the firing chamber explodes from overpressure. $\endgroup$ Nov 27 '21 at 17:28
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The simplest solution is... a solution of your explosive in a solvent that evaporates quickly.

For example, nitrocellulose (guncotton) is soluble in acetone (a 25% solution was historically used as a lacquer), acetone is a solvent with a high vapor pressure (i.e. evaporates quickly). If you want a quicker evaporation, you may use a lower pressure (but run the danger of creating some porosity).

Guncotton was originally made from cotton (as the source of cellulose) but contemporary methods use highly processed cellulose from wood pulp. While guncotton is dangerous to store, the hazards it presents can be reduced by storing it dampened with various liquids, such as alcohol. For this reason, accounts of guncotton usage dating from the early 20th century refer to "wet guncotton."

The power of guncotton made it suitable for blasting. As a projectile driver, it had around six times the gas generation of an equal volume of black powder and produced less smoke and less heating.

Searching for "nitrocellulose acetone max concentration" got over The Solvation of Nitrocellulose in Acetone-Water mixture - Royal Society of London 1933

... Acetone is an excellent solvent for nitrocellulose. It forms solutions with materials within a wide range of viscosity and degree of nitration ; its vapour is rapidly absorbed.

They managed to completely dissolve 14g of nitrocotton in 200cm^3 acetone/water at 93% (acetone swells nitrocellulose to the point of making it some viscous suspension, but they didn't consider this a "solvation")

See also US Patent 3,447,983/1969 - ACETONE TREATED NITROCELLULOSE-BASED PROPELLANT AND PROCESS (as PDF).

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    $\begingroup$ ammunition is heavy and a solvent is added weight. Weight that is just waisted away to try and make your propellant liquid. Though it is an option where few exist. (14g of nitrocotton in 200cm^3) that is about 14g in 200g solvent $\endgroup$ Nov 27 '21 at 9:50
  • $\begingroup$ @PostlimFort if the acetone were to be pure, that's 14g completely dissolved in 156.8g - that acetone had water, which impede the dissolution. You had an example in which nitro-lacquer is 25% by weight and used even today (mainly guitar aficionados). Also, colloidal suspension in higher concentration can be obtained - you can pour them, albeit one should expect a way higher viscosity $\endgroup$ Nov 27 '21 at 10:26
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    $\begingroup$ nitro-lacquer is better weight wise but still 75% is wasted. i should have explained that more. 14 g in 200g has you wasting about 93% if i had used the more correct value you'd come out to wasting about 91%. that is why i didn't bother looking up those values. colloidal suspensions are fascinating but the viscosity means that you would need to treat them as a solid inside a sidearm with the added complications of it actually being a liquid. $\endgroup$ Nov 27 '21 at 10:52
  • $\begingroup$ 93% acetone/water is more dense than you may expect. i use a rule of thumb at work for the density of such a liquid. i take the weight of the bulk liquid (156.8) then i add the weight of half the other liquid and i take a weighted average with the other half. > ((157+7)*96.5+200*3.5)/100=165 than for a proper percentage calculation you need to take 14 out of that 179 (165+14) $\endgroup$ Nov 27 '21 at 11:42
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    $\begingroup$ Do you really need to sit and wait for the "paint" to dry between shots? I suppose it would make battle a bit more meditative and soothing to some people. $\endgroup$ Nov 27 '21 at 11:44
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A heating element can used in the magazine to keep it just above melting point. Then it would cool solid in the barrel. This can and probably will be a slow-ish process. Problems may arise if the weather is warmer than expected.

You may have a substance that is kept liquid by keeping it under low pressure. Were it would solidify under normal pressure. But that just won't leave the magazine.

A solvent may work. Adrian has a great answer for that option. (acetone for gun cotton) Of note is that ammunition is heavy and a solvent is added weight. Weight that is just waisted away to try and make your propellant liquid. Lindybeige a youtuber had a video a while ago about why soldiers carry clips in stead of magazines. The main takeaway was that magazines were to heavy and cumbersome.

As for a chemical reaction. There isn't any mechanism. For an application like this you can consider the air to have 3 compounds Nitrogen, Oxygen and trace gas.

Trace gas is a mixture of a whole bunch of different gasses all of which are only to be considered when a high amount of purity is desired. Or for a slow processes. There is just to little of it available.

Oxygen is one of the elements in the propellant. It is often added in a solid form. Not solid as in frozen but as a part of a molecule that is solid. If your propellant would react with the Oxygen in the air then it would explode, or more likely just burn away. So the Oxygen in the air is more of problem than a solution.

The Nitrogen in the air is a little more interesting but of no use for this. Nitrogen as an element is also used a lot in modern propellants. It is used to hold most of the chemical energy to power the explosion. In the propellant each Nitrogen atom is held away from the others by having a "spacer" element in between. When the explosion happens all of these Nitrogen atoms should form pairs and release a lot of energy by doing so. So far so good. Only the compound formed this way is the same as the one in the air. Meaning that the compound in the air already has reacted. And due to the lager than usual amount of energy released to become that form wouldn't react with anything tame. So if your propellant would react with the Nitrogen in the air then it would be spent just by becoming solid.

Edit If you allow the use of 2 liquids then there may be some more solutions. The mechanism will probably be similar to multi component glue.

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  • $\begingroup$ This was going to be my answer. Candidate molecule is naphthalene! en.wikipedia.org/wiki/Naphthalene_locomotive $\endgroup$
    – Willk
    Nov 28 '21 at 18:24
  • $\begingroup$ @Willk it might work. the physical property's are good. it still needs an oxidiser. you may try making gunpowder with it. just switching out the charcoal for naphtalene. see it it can stay mixed or that the sulfur and saltpeter settle to the bottom to quickly. ---------------------------- for an explosion you need an oxidiser and a fuel mixed together thoroughly. unfortunately they usually mix about as well as oil and water. you can make a 'mayonnaise'. that would see them mixed good enough for a explosion. $\endgroup$ Nov 28 '21 at 18:52
  • $\begingroup$ I am thinking about an internal combustion engine but instead of a piston the cylinder would throw a projectile. It could have multiple barrels, Internal combustion engines breathe air. The liquid fuel can be atomized which is a good way to get air / fuel interfaces. $\endgroup$
    – Willk
    Nov 28 '21 at 19:39
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    $\begingroup$ @Willk interesting idea. but remember OP's question. we are trying to make something that is liquid in the magazine and solid in the barrel. $\endgroup$ Nov 28 '21 at 21:01

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