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My scifi settings armory is filling up with many interesting weapons. However, while energy weapons, Gyrojets, Tangler guns and the like outperform slugthrowers in special applications, concentional slugthrowers still reighn supreme as all-round guns. The wide array of special ammunitions make them even more versatile and useful. Sure, a Gyrojets fire rate and heat management in vacuum or an IR-laser-snipers stealthyness can't be achieved, but carrying 15 different kinds of special ammunitions allows the slugthrowers to keep up by being versitile.

I do want to mix up slugthrower design a bit to drive home that this is the future. While handheld rail- and gauss guns exist, they are more of a novelty like plasma guns, than actually useful weapons. What caught my attention however are combustion light-gas guns. These guns use oxygen-hydrogen or hydrocarbon mixtures to propell their projectiles. The wiki article says that muzzle velocities of 3 km/s are achivable, but that they have a poor accuracy at artillery ranges. This is bad for artillery applications, but doesn't really limit the use of these a vehicle mounted or handheld guns.

Advantages and Disadvantages

  • the fuel, be it hydrogen, methane, ethanol or a higher order hydrocarbon, is readily available and cheap; especially spacecraft will have a pretty much infinite supply of "gun gas"

  • the fuel is harder to handle; while hydrogen gas has an amazing gravimetric fuel density, the volumetric fuel density is atrocious; carbon containing fuel alternatives metigate the issue a bit, but are still hard to handle; however advanced material science could solve this, a fuel cartouche made of graphene will make handling high pressure hydrogen much more pleasant

  • increased muzzle velocity allows one to either go for more damage or to reduce projectile mass while keeping the damage on a similar level, this in turn would increase magazine capacity significantly

Are combustion light-gas guns capable of competing with conventional solid fuel ones, given the technology matures significantly? What would be other advantages and disadvantages? If they can't compete, what would have to happen so they can?

Edit1: This is a conventional gun, except that the gunpowder has been replaced with explosive gas. It shoots normal bullets, not Gyrojet Rounds.

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  • $\begingroup$ They would probably be pretty good for use in space. but if used on the ground, the bolts would fizzle out pretty quickly, depending on the atmospheric pressure and atmospheric composition. $\endgroup$ – Greenie E. - Reinstate Monica Nov 4 '19 at 20:23
  • $\begingroup$ @GreenieE. What? Why would they fizzle out? They are like normal bullets? $\endgroup$ – TheDyingOfLight Nov 4 '19 at 20:31
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    $\begingroup$ I think the biggest problem would be practicality, as bullets that carry their own energy are just so simple. Even an electric weapon only needs to be recharged (laser gun or whatever). A gas gun would need two gasses, bullets, and probably electrical power too; which would result in quite an annoying reloading procedure. $\endgroup$ – Dragongeek Nov 4 '19 at 20:42
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    $\begingroup$ Seems like a bit of weird technological dead-end to me. $\endgroup$ – Starfish Prime Nov 4 '19 at 21:15
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    $\begingroup$ @RonJohn it doesn't even have a cool name or even a half-decent ominous acronym. Now if the OP had suggested scram cannon on the other hand, I'd be right behind that... $\endgroup$ – Starfish Prime Nov 5 '19 at 17:35
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There may be special applications in which a gas-fueled gun would compete with "conventional" types, but it'll never be more than a niche weapon at artillery scale, and never competitive at the small arms level.

Why?

First, gas has a very low energy density relative to the propellants now in use -- there's a lot more energy in redox combustion per gram, but it's very hard to get a gas anywhere near the grams per cubic centimeter density of ordinary single- or double-base smokeless propellant. By the time you manage it, your cartridge will be effectively a high pressure tank full of fuel and oxidizer, and will still be several times the size and weight of a ballistically comparable smokeless cartridge.

For a battleship gun, using external tankage and very long barrel with "pump gun" feeds, gas fuel might produce a small improvement in range over a conventional naval rifle -- but at the cost of hugely increased complexity (= failure rate) and the general fragility that goes with an extremely long barrel. It's been demonstrated a number of times that fixed, land-based pump guns are impractical (as is any other kind of fixed artillery, with the exception of shore defenses, even if you're limited to WWII tech).

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They're probably not very useful as small arms.

Firstly, they seem to need longer barrels than their conventional propellant equivalents in order to develop those high muzzle velocities. The artillery prototypes are 50-100% longer than the regular kind. That's a really awkward thing for infantry to have to tote around... even for snipers that's something of an inconvenience.

Secondly, it isn't obvious that lightweight, high-velocity projectiles are useful in small arms... the flechette rifle fad of the 80s fired long, fin-stabilised rounds at substantially higher velocities than conventional rifles (1400m/s 1.6x42mm dart vs more like 900m/s for a 5.56mm NATO round at ~23mm long) and were found to have issues with deflection in rain and penetrating light cover like vegetation. That was a long time ago of course and technology marches on, but those worries haven't gone away yet and may be insurmountable.

Thirdly, the big advantages of such a weapon, ability to synthesise propellant in-situ, variable muzzle velocity or constant muzzle velocity with different mass projectiles, is not particularly useful for infantry either. Great for naval artillery, and the potentially long range is very useful even for conventional land-based artillery. Increased range isn't useful for most infantry engagements, and given the possibile sensitivity of the round to environmental problems it may not be achievble.

For vehicle use the downsides are largely surmountable, but it requires that vehicle mounted railgun and coilgun technology does not march on appreciably. The CLGG would at least not require a large power plant to operate. The innaccuracies for artillery use are also readily surmountable. I'm not sure it has any relevance in space combat at all; its velocity is too low compared to rocket propelled projectiles, and there are too many competing alternatives for it to be plausible that all of them are worse than this one.

Seems to me like these will make for good artillery weapons in the short-to-medium term, depending on how electromagnetic guns pan out. Not much more than that, though.

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  • $\begingroup$ Military air rifles were a thing that existed. Austria used them for decades in the late 1700s-early 1800s, according to Wikipedia. en.wikipedia.org/wiki/Girandoni_air_rifle $\endgroup$ – nick012000 Nov 5 '19 at 14:30
  • $\begingroup$ @nick012000 and they generally offered to few upsides to counter their downsides for the vast majority of users. I'm not sure how the girandoni is relevant to theoretical CLGG small arms, though? $\endgroup$ – Starfish Prime Nov 5 '19 at 14:50
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Automated paintball shooting riot control robots.

These large and imposing robots are armed with paintball cannons which are fired by an onboard tank of propane rather than compressed air. Rotating barrels like a Gatling gun each get a paintball and then the combustion chamber detonates, firing the ball with 2-3x the power of a standard paintball rifle (and a cloud of flame!). The paintballs are full of paint and capsicum; they are fairly solid and they hurt. Some contain poison ivy juice instead of capiscum. Some contain sodium butyrate.

A flame-spewing robot is more imposing than a small riot control cop with a rifle that shoots invisible things, if he shoots. Flying balls that leave giant irritating splotches can easily be appreciated for what they are, and they are many, every second, and leave many nasty burning, itching, stinking welts. The robots can carry a lot of paintballs and one big tank of propane is enough to propel tens of thousands of them.

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The problem is the amount of gas that can be generated in a given volume and the speed with which it can be generated.

Using a compressed gas as a starting point is better than an uncompressed gas but is far worse than using a solid to create the gas because gases are so much less dense than solids.

An explosive solid (or liquid) will generate a very large volume of ultra-compressed gas in micro seconds. In addition when most of the chemical bonds break and rearrange themselves in a more stable configuration a great deal of energy is released making this ultra-compressed gas very hot as well.

A gas is simply not going to be as good an explosive as a solid because it is so much less dense.

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While it is certainly possible to achieve extremely high velocities using exotic weapons and equipment like gas guns, gauss guns or rail guns, the probability of them being common or effective military weapons is actually rather small. This is because the primary issues for military weapons revolve around the reliabilty of the weapons system itself, and the associated logistics involved in using the weapon.

Consider a 155mm howitzer, a fairly common calibre among Western armies. The gun is essentially a steel tube with a movable cap on one end, a design first demonstrated in the 1400's. The sorts of weapons modern armies use simply take the tube capped at one end principle with about 600 years of refinement. Although it takes a great deal of industrial know how to make reliable and accurate artillery, the end result is pretty easy to use and soldier proof.

The ammunition is similar, highly refined over centuries of development, producable in mass quantities for a relatively low cost and compact and easy to transport, store and quick to utilize in the field once the transport or shipping containers are opened.

A simple gas gun, on the other hand, not only has more parts than a conventional cannon, but requires a very high degree of machining and ultra fine tolerances. If the bursting disc isn't inserted right, or was damaged in transit, then the crew will go through an elaborate setup, fill the tube with hydrogen, put the charge behind the piston and instead of a shot screaming out the barrel at 8km/sec, you have a high pressure detonation right in the gun crew's face. And in addition to ammunition, they need bursting discs, tanks of high pressure gasses (or cryogenic dewars of liquified hydrogen and oxygen), spare pistons or piston rings, and a host of other stuff, all which needs to be carried to the battlefield and delivered at the right time and place. The combustion light gas gun has similar issues, even if the detailed parts and supply lists are different.

Rail and Gauss guns fail as battlefield weapons because of their immense energy requirments, and may really only work well on ships, and to a lesser extent aircraft which have high energy power plants which can be tapped and a nice cold sink to dump the waste heat into (ocean water or air passing over heat exchangers build into the wings and fuselage). Spacecraft might be a good third choice because they can access high energy via rocket engine like generators (either turbogenerators spinning high speed generators or MHD generators), nuclear reactors or possibly beamed power from a solar array. Spacecraft can also carry or unfurl large radiator surfaces to deal with the waste heat.

Gas guns could work as space weapons, but since a spacecraft needs to carry all its supplies aboard, there will be a limit to the amount of propellant which can be carried. A ship the same size can use the space devoted to fuel and oxidizer tanks for the gas gun to house a larger generator and more slugs to fire through the rail or gauss gun. And since even a 1980 era design like "HAVE STING" has a muzzle velocity of 15 km/sec, it will outperform any gas gun.

So gas guns (either simple two stage weapons or combustion gas guns will likely be bypassed as being overly complex, not soldier proof and heavy logistical burdens.

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  • $\begingroup$ I made the same intial mistake as you here... the unfortunately named "combustion light gas guns" are single stage weapons that use gas-phase propellant, and are almost entirely distinct from regular light gas guns other than that some of them use hydrogen in their propellant mix. $\endgroup$ – Starfish Prime Nov 5 '19 at 17:26
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Variable power.

If you use gas on a munition and can adjust the amount, you can get the Judge Dread's gun. Hard walls? More fuel.

Soft walls and want to avoid over penetration? Less.

Set your gun to hold the fuel and the ammo only the tip of the projectile on the magazine. Lets you carry far more ammo per clip. The magazine can be safer to carry around since it only holds inert tips.

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    $\begingroup$ This question isn't about Gyrojets, it is about conventional slugthrowers using gas for the explosions. $\endgroup$ – TheDyingOfLight Nov 4 '19 at 20:32
  • $\begingroup$ Totally missed the point, will edit it . $\endgroup$ – Gustavo Nov 4 '19 at 20:34
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Absolutly no

You see, the "speed of explosion" and thus the theoretical maxumum of bullet speed (relative to gun) are

  • for morden gunpowder 2 500 m/s
  • for gas 160 m/s

Real bullet speed is about 250-700 m/s, about 10-25% of theoretical maximim. Things are non-lineer here (less gas speed - more %), so for gas 30-50% is achivable. But this will only give bullet speed 50-80 m/s. Its less than BB gun! It means that just compresed gus without explosion would be more effective. And you can't use "combined" effect. Speeds do not add up here.

I personaly experimented with natural gas and air mixture in pneumatic gun (do not repeat it - its dangerous!!!). It gave no mesarable effect (only louder sound).

So no - it better to use some form of modern scorpio (it is more energy efficient than crossbow), than gas-gun.

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    $\begingroup$ Guns that fire using compressed air exist, and absolutely can kill. They were even experimented with by various militaries at one point. $\endgroup$ – nick012000 Nov 5 '19 at 14:23
  • $\begingroup$ Uh, CLGGs with >2km/s muzzle velocities already exist, albeit in prototype form. Hell, even pumpkin cannon manage significantly higher muzzle velocities than 160m/s. Even if the OP were only talking about compressed gas guns (which they very definitely are not) then hydrogen-filled light gas guns exist that have muzzle velocities in excess of 8km/s. $\endgroup$ – Starfish Prime Nov 5 '19 at 17:34
  • $\begingroup$ @StarfishPrime, nick012000, OP question was not about compressed gas guns, but about compress&combustion guns. And my answer is all about "combustion" part. Yes compressed guns can be lethal (if you luky) or achive high projectile speed (within laboratory conditions), but this is another question to answer. My answer is that "combustion" doesn't add much to "compressed" part - it is to slow. Compressed gas flow outruns firefront. But thanks, I will try to clarify this point. $\endgroup$ – ksbes Nov 6 '19 at 6:24

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