In my world, there is no access to modern or traditional gunpowder. I was wondering if a dedicated technician with a modern shop (cnc machines, and a surplus of parts and raw material, etc) would be able to develop a bullet the fires from compressed air that is in the same cartridge as the projectile. Issues I am not sure of are: How big would the cartridge have to be to push a 30 caliber bullet at lethal trajectory? and how thick would the air chamber have to be to hold that pressure?
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1$\begingroup$ what is the difference between this and an air-rifle? Those can kill and it is easier to pump air into a big container than tiny cartridge $\endgroup$– aaaaa says reinstate MonicaCommented Apr 23, 2019 at 17:54
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3$\begingroup$ You would never have to pump your gun if you left with enough bullets. I find that an air rifles biggest weakness is the limited amount of shots they can fire. So if there is a large fight where the mechanic I described has to fight more then ten enemies, he would be able to without having to retreat and pull out a bicycle pump to charge up. $\endgroup$– AlexCommented Apr 23, 2019 at 17:58
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$\begingroup$ Air rifles are usually charged by breaking and closing again. This bratty kid shows you how - youtu.be/AI2b5n4nkCg - Two or three pumps are usually more than enough. No bicycle equipment needed. Are your self-contained bullets loaded individually or is this an automatic weapon? $\endgroup$– chasly - supports MonicaCommented Apr 23, 2019 at 18:12
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1$\begingroup$ @chaslyfromUK - I doubt three pumps compresses enough air to push a 210 grain sphere of lead at 500fps. - OP, your objective should be lethally propelling lead with air, not trying to make something disposable that I wouldn't want to be in the same room with... "Contents Under Pressure." $\endgroup$– MazuraCommented Apr 24, 2019 at 9:10
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1$\begingroup$ How do you create a world with organic chemistry and not discover gunpowder ? No systematic industrial chemistry could avoid finding it. Long before you get to make useful high precision CNCs and compressed air you will discover gunpowder. Gunpowder exists since the 9th century and compressed air to the extent you required would not be available until maybe the 17th century. The cannon was developed in the 12th to 13th century. $\endgroup$– StephenG - Help UkraineCommented Apr 24, 2019 at 11:13
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3 Answers
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Combining the projectile directly with the air chamber and a single use valve is possible, (and exact sizing would depend on materials and valve design) but it is vastly inferior to a bulk storage tank design that is separate from your projectiles:
- Bulk tank storage means that you need 1-2 valves to fire potentially dozens of rounds at lethal velocities in place of at least one valve per shot.
- Reduced valve count translates into reduced maintenance (and potentially increased safety)
- Handling the independent projectiles does not introduce risks of damage or leaking charges.
- Bulk storage allows option for consistent regulation and reduces the risk of shot-to-shot variance due to differences in cartridge pressures.
Consider the Girandoni air rifle - each cylinder allowed approximately 30 rounds to be fired, and the soldier was typically issued two spares. The accompanying (ball) ammunition occupies a very small and highly flexible volume - This translates into ease of carrying and handling.
Contrast this to the (non-lethal) options seen in paintball and the obscure 'goblin' shells that held one paintball and an air charge to fire it: These proved unreliable, inconsistent, and took up far more space than a CO2 cartridge beside the paintballs that could be fired with one. And the small 12 gram CO2 cartridges weigh more and take up more space than what could be achieved with a larger bulk tank.
For handling and ergonomics, a mixed Tank+Magazine design may be what you're really after - A single bulk tank with capacity to hold a charge for multiple rounds, and a magazine of ammunition that the tank can reliably fire.
(Answer can be improved with exact numbers by myself, or another member, if needed/someone gets around to crunching the numbers - Material options and exact bullet size/weight would need to be established for accurate figures.)
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1$\begingroup$ I feel like there has to be some material that we have developed that would allow for high pressure air storage without being to bulky. The Girandoni has obviously proven its worth, but I can't help but think their must be a better way. A weakness I see is the reliance on a tank of air. even if you carry two a trip and fall might render that who supply worthless if the nozzle was to break, or a leak was able to form. $\endgroup$– AlexCommented Apr 23, 2019 at 18:30
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1$\begingroup$ The Girandoni was rugged enough for Lewis and Clark on their mission of exploration. It was good enough to come under an attempted ban during the Napoleonic wars, because it had the accuracy and power to kill, but didn't give away its location with clouds of smoke -- and the guns stood up to the handling of soldiers in war, albeit an elite unit. $\endgroup$ Commented Apr 23, 2019 at 18:43
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2$\begingroup$ @Alex you would be extremely hard pressed to approach the consistency and weight/volume efficiencies with a per-cartridge charge. You may get closest if you could make the projectile itself the tank, with a primitive burst disk 'valve', but then every round is a quality control nightmare and vastly more difficult to produce in bulk. - Any improvement you make with materials for the single use cartridge has More improvement in bulk storage. $\endgroup$ Commented Apr 23, 2019 at 19:09
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3$\begingroup$ The rocket equation places strict limits on the mass ratio of any projectile powered by exhausting propellant. Space-going rockets are typically more than 85% propellant by mass, or 1% payload when you account for structural mass. An air gun wouldn't need to work nearly as hard against gravity, but compressed air (the other two parameters of the equation) is a pretty inefficient propellant compared to rocket fuels. Even if your bullets were 50% propellant by mass that's a significant loss of kinetic energy after even short flights. Hypergolic propellants might be a more believable solution. $\endgroup$ Commented Apr 23, 2019 at 20:48
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2$\begingroup$ @MatthewGauthier : the rocket equation is completely irrelevant here, because the bullet in this case doesn't have to carry the propellant to be used while in flight. The small compressed air canister can be in the cartridge which stays in the gun and will be ejected later. It doesn't have to fly with the bullet, exhausting the air while in flight. $\endgroup$– vszCommented Apr 24, 2019 at 6:27
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The Brocock Air Cartridge System seems to fit your description. The cartridges are loaded with an airgun pellet and primed from a pump or diving tank. They are designed to resemble and function as close to real firearm cartridges as possible, rather than being designed for the kind of power you describe.
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$\begingroup$ I was going to say, this already exists! A friend of mine had a gun like this as a teenager. $\endgroup$ Commented Jul 7, 2023 at 12:29
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I've actually designed (though not built) a cartridge that works like this. By my calculation, and by comparison to conventional precompressed air guns, I think a cartridge the size of a .30-06 round holding pressure similar to that used in PCP air guns should be able to propel a lightweight .30 caliber bullet (say, 100 grain or lighter) at close to the speed of sound, or a heavier bullet at somewhat slower velocity.
This is adequate for hunting small game (squirrels, rabbits, etc.), or marginally adequate for self defense against unarmored assailants -- ballistically, it would be similar power to a .32 ACP handgun cartridge, possibly as good as a .32 H&R Magnum (the former fires an 88 grain bullet at slightly subsonic speed; the latter fires a 100 grain bullet at supersonic muzzle velocity). It would be "single shot" in a handgun, but in a rifle sized arm could be loaded from as magazine, as was done starting in the 1890s with powder weapons.
This could be scaled up some -- a larger air capacity cartridge will give more velocity and/or propel a heavier bullet -- but there's a limit to how big it's practical to make your cartridges. If you need something like a .50 BMG cartridge to hold enough air to propel a 180 grain .35 bullet at Mach 1.5 (i.e. enough power to reliably kill deer at moderate range), the ammunition bulk is likely to limit how many rounds you can carry.
answered Apr 23, 2019 at 18:23
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$\begingroup$ If the projectiles are too bulky I could see possible use in a mounted weapon. There would be no need to move the gun, and the air would allow for less heat to affect the weapon. $\endgroup$– AlexCommented Apr 23, 2019 at 18:32
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$\begingroup$ @Alex well tbf if it's a mounted weapon it would probably benefit from a tank (which you could make as durable as you want, since it's mounted anyway) $\endgroup$– somebodyCommented Apr 24, 2019 at 7:15