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I got a little question: how would it effect firearms if they would not be based on gunpowder but on gasoline like flammable liquid? What I think would happen: -because the liquid would need to be a mist to be able to explode and this would not be really storable in a shell so guns themselves would need a ignition chamber, a nozzle to spray the liquid in the chamber, a inbuilt tank to store the liquid and a mechanism to pump the liquid. Because they need all of this they would become bigger and therefor heavier and therefor less suitable for infantry

-because they have all those extra mechanisms they would have a low fire rate even in modern times and increase the cost of maintenance

-gasoline is way weaker then gunpowder so the shots themselves would not be that strong and could potential even be blocker by a steel armor or shield

Do you guys have any more ideas of things that would change or think that I got something wrong?

Thank you all in advance!

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    $\begingroup$ (1) Modern (= after 1900) firearms do not use the kind of gunpowder invented by the Chinese 1200 years ago. (2) Gasoline was invented about a thousand years after gunpowder. That is, to make gasoline one needs technology very very much more advanced than what one needs to make gunpowder. (No, one cannot simply boil petroleum and obtain gasoline. It is much more complicated than that and involves advanced organic chemistry.) When gasoline was invented they already had modern explosives and "smokeless powder" propellants, such as cordite (English) and Poudre B (French). $\endgroup$
    – AlexP
    Commented Nov 26, 2021 at 21:05
  • $\begingroup$ @AlexP thank you for this comment but this is not really getting the point of my question: I am interested in how a gun would need to be modified in order to work on gasoline or gasoline like liquid. A scenario could be a (perhaps modern) fantasy setting in which solid propellants are not available. I'm sorry if I didn't bring this across this way and can change the question if there is a better formulation. $\endgroup$
    – Mr M
    Commented Nov 26, 2021 at 21:44
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    $\begingroup$ To burn gasoline you need an oxidizer. Where does the oxidizer come from? (Gunpower and other propellants work because they contain both the fuel and the oxidizer. Gasoline needs about 15 times its weight of air to burn; for example, to burn 1 gram of gasoline you need 15 grams of air, which would be about 15 liters or 4 US gallons.) $\endgroup$
    – AlexP
    Commented Nov 26, 2021 at 22:45
  • $\begingroup$ @AlexP much lesser in an atmosphere richer in oxygen and a lot less volume if you compress it (so not necessary a clear cut it cannot work in any conditions). Besides, ignition for things under pressure is easier - you are half-way to the activation energy of the reaction by applying pressure alone. $\endgroup$ Commented Nov 27, 2021 at 3:24
  • $\begingroup$ Kerosene, natural gas, and propane are all higher in energy density than gasoline, which started off as a kerosene byproduct. They might be more likely alternate combustion sources. $\endgroup$ Commented May 14 at 22:58

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It won't work as simply as you describe, because you need an oxidizer (and enough of it) for the gasoline to burn.

Now there is one way to get it to sort of work. It's a fairly well known phenomenon among air gun users, called dieseling. Some kinds of air guns are cocked by compressing a spring. When the trigger is pulled, the spring is released, and compresses the air in front of it, pushing the pellet out the barrel. Now as we know from basic physics, compressing a gas also heats it. If there is some lubricant between the pellet and the jet of hot air, it may ignite, creating even more pressure, and thus shooting the pellet faster. This is exactly the same principle used in diesel engines. See e.g. https://airgunplanet.com/what-is-airgun-dieseling-and-how-dangerous-is-it-really/

So you could perhaps design a gun that works off this principle. However, you'd have to have some way to compress the air (for instance by the user pumping it), a mechanism to inject just the right amount of fuel per shot, and perhaps a spark plug to make sure it ignites. So your gasoline gun is going to start out being rather complicated, high-maintenance, and fault prone, and probably too heavy for a personal/infantry weapon.

So let's re-think this a bit. Your gasoline (or diesel) gun is basically the same as a one cylinder gasoline engine. You need a piston to compress the air-fuel mixture, and valves to direct the combustion gasses behind the projectile. So you add another cylinder which is nothing more than a gasoline engine. This provides power to keep the whole thing turning. Add a mechanism to feed projectiles to the firing cylinder, and you have a gasoline-powered machine gun.

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Gasoline would not work well as a propellant, because its tendency is to burn rather than explode. (They're not called internal explosion engines, after all.) In technical terms, this means that the shock front of the combustion - the dividing line between "burned" and "not burned" - moves slower than the speed of sound.

You can see this for yourself by looking at movie explosions vs. real life explosions. In the movies, what you mainly see is burning gasoline, which produces very dramatic globes of golden flame that visibly blossom and spread. In real life, there's very little flame, and the shockwave of an explosion travels faster than you can easily see.

Mechanically, this translates into lower brisance, or peak pressure. This is an important measure of how well a propellant propels because it measures how much energy you can quickly impart onto a projectile. It's no good having a lot of potential energy if it takes a long time to be expressed, or if your projectile has already left the barrel (leaving the gasses free to expand behind it). So while gasoline is a great fuel for things that need to move predictably over time, like engines and turbines, it's not great for applications that need a lot of force right now.

I suspect that an air gun would be your best bet. Weapons like the late-18th-century Girardoni rifle were deadly, highly accurate, and had few of the drawbacks of conventional black powder guns (such as smoke and vulnerability to dampness). Its drawback, and the reason it was eventually passed over, was the insane amount of pumping needed to refill its air reservoirs, and the difficulty in manufacturing them, limited the number of shots a soldier could bring to the front. A small, man-portable gas compressor could allow company- or squad-level resupply right on the front line, giving you great longevity.

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    $\begingroup$ But the propellants used in guns burn rather than exploding. The burn rate of gunpowder has to be matched to barrel length: using fast-burning pistol powder in a rifle can wreck it. Indeed, I recall at least one mystery novel where the murder was carried out by replacing the powder in a shotgun shell with a high explosive. $\endgroup$
    – jamesqf
    Commented Nov 27, 2021 at 19:13
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Gasoline actually has some of the highest energy densities that can be achieved with modern technique. A gun works by creating a small directed explosion that propels a projectile with rapidly expanding gasses. If an oxidizer is added to the fuel to create the desired amount of explosive sensitivity and ratio of oxidizer/fuel one can achieve the explosive mixture without the need to modify the guns nor the bullets too much at all, it just depends on whether or not your society has discovered this.

Having said that, otherwise using a non-oxidized liquid propellant for your gun would probably only result in some burns on the hand of the would be shooter when burning liquid inevitably leaks all over his hand. Maybe your society is more advanced on the area of incendiary devices if they aren't adept in using oxidizers for whatever reason. It would change the face of battles completely in your setting.

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Flamethrowers

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If all you have is gasoline, then your most practical weapon will be a flamethrower. You don’t have to worry about exploding the stuff, or projectiles. Just use a gas-powered compressor to fill up a tank of propellant, attach a tank of gas, stick a lighter in front, and go to town. Ideally, thicken the gasoline into a napalm like substance.

Unlike the cheap, gas-based flamethrowers, a liquid flamethrower boasts superior range, devastating effects on target, and completely ignoring any infantry armor. The WWII era M1A1 flamethrower was man portable, effective to 45 meters, and worked very well against armored pillboxes and fortifications.

The main effect this would have on warfare is hard to predict. Historically speaking, even experts struggle to fully predict the effects of new weapons in combat (see WW1). One thing is for sure, it’s going to be very unpleasant. Napalm burns are horrific.

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It common in spring piston air guns for the air between the piston and the projectile to briefly become very hot during firing.

Hot enough, in fact to ignite fuel, such as gasoline, mixed into that air.

To modify a spring piston air gun to use gasoline, several changes would be needed.

First, a thicker walled barrel and firing chamber, to not be damaged by the firearm-like high pressure.

Second, metal piston rings instead of the typical rubber or plastic seal.

Third, a fresh air intake, since normal springer guns have nothing to ensure air flow (scavenging).

Fourth, a carburetor or fuel injector.

Sixth, a sparkplug. Although the gasoline / air mix can detonate, it will be less harmful to the gun parts and produce a more reliable amount of thrust if it deflagrates instead.

If you have no sparkplug, and instead rely on spontaneous combustion due to compression ignition, the timing of the ignition will depend on the temperatures of the air, the piston, the walls of the firing chamber, etc.

Seventh, a gas spring instead of a steel spring. Optional.

Eighth, some repeating mechanism. Optional.

A gasoline fueled gun will have more in common with a gasoline two stroke engine than with any real world gun or air gun.

That's not a bad thing, but it means that you might want to talk with gear heads instead of gun nuts for insight.

Also, notice the ammunition would be shell-less, which means that the you can omit the typical shell ejection mechanisms which gunpowder firearms have.

It's even primer-less, and considering what's in gun primer, that's a great thing.

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Gasoline has 13 times the energy of modern gunpowder. It's not weak at all. The problem is 1. Oxidizer. It needs a pump for ambient air. 2. Slightly different amounts of gasoline introduced to the combustion chamber depending on ambient temperature. Higher temperature means less density. 3. Velocity. Gasoline has half the detonation velocity. So a heavier bullet is needed to deliver the same energy since gasoline just can't push a smaller bullet fast enough.

  1. It could still work though. As far as oxidizer goes, recirculate some of the exhaust gases as an air compressor. Gas pistons and recoil springs are already enough to cycle the round and eject the case.

  2. Without complex electronics there's not much you can do about slight density differences. It's more then sufficient for hunting or militia.

  3. Since the fuel and bullet are separate there's no need for a case. Just make a longer bullet for the same caliber. It'll go slower but hit just as hard.

Ignition isn't hard either. A gas piston spring used for kids can compress air fast enough to reliably cause ignition. A separate spring directly connected to the trigger can force a small amount of fuel fast enough to turn it into a mist of forced past a thin grate. Firearms typically use 1 watt hour or less at a time. So the mechanism needed to inject fuel as well as ignite said fuel can easily be supplied by average finger strength pulling a trigger.

The biggest issue is gasolines shelf life. 6 months to a year tops. Modern day smokeless powder has a shelf life of years.

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