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Air rifles have come a long way. Starting with crude devices that can barely launch a projectile to the Girardoni air rifle to modern examples like the Air Force Texan or the Air Venturi Dragon.

There is however an errant cousin that has fallen to the way side when compared to more traditional pneumatic firearms, the Giffard gas pistol. It differs from other pneumatic weapons by housing a container of liquid carbonic acid instead of a pressurized gas.

What I'm wondering is if we were to make a air gun that utilized carbonic acid, how well would it perform when compared to modern air rifles? Would there be a difference in performance at all?

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    $\begingroup$ (1) Carbonic acid as such can exist only in solution. (2) But in the olden days before ultra-modern terminological fussiness, carbon dioxide was called carbonic acid. (Or, when they wanted to be more precise, anhydrous carbonic acid.) (3) Overall, the pistol had a container containing liquid (or supercritical, depending on termperature) CO2, not all that much different from the small old school CO2 steel containers (for making soda water) which were commonplace some forty years ago. (4) One thing to be careful with is that CO2 becomes very cold when it expands. $\endgroup$
    – AlexP
    Jun 30 at 7:28

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Minor Frame Challenge

It differs from other pneumatic weapons by housing a container of liquid carbonic acid instead of a pressurized gas.

The Gifford Gas Pistol does not house a container of liquid carbonic acid, it houses a container of water that you put a solid tablet of H2CO3 (Carbonic Acid) into and then shake. This causes basically the same reaction you get from shaking a soda where the dissolved H2CO3 breaks down into H2O and CO2 releases a CO2 gas... but also like shaking a bottle of soda, the actual power you get out of such a reaction is not super impressive. The H2CO3 will only break down to a point. Trish's answer already explains this part very nicely so no need to restate that part.

How it compares to Liquid CO2

This is intrinsically different than a liquid CO2 gun. Liquid CO2 is made by mechanically compressing and cooling CO2 gas until it becomes a liquid. This allows for much greater pressures, and therefore higher mussel velocities.

While technical information on such a rare class of gun as the Gifford Gas Pistol is exceedingly hard to find, according to this source, the Gifford Gas Pistol was only lethal within a range of 20ft. and only if you landed a head shot suggesting a weapon no more powerful than a low quality .22 pistol.

Even Better Modern Alternatives

Either way, CO2 guns are not really all that impressive with many of the most powerful CO2 weapons toping out at about 760 feet per second. This is actually way less than a high end compressed air style riffle which can get up to about 1600 FPS. This is because CO2 expands slower than plain old air. So CO2, despite its popularity in non-leathal weapons like paintball guns is not an ideal gas for lethal air riffles. If you really want to make a powerful air riffle, you would use a compressed noble gas like Helium or Neon. Helium has such a high rate of expansion that Helium Gas guns are used to simulate micro-meteor impacts by NASA. Neon has a rate of expansion more similar to air, but it has the highest expansion ratio out of all liquified gases meaning you can miniaturize your air tank about 2 times as much as with helium and 3 times as much as CO2 which could make it more ideal for portable compressed air weapons.

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There is little to no difference

When I last went to a gunshop to get a few new arrows (because they are the only ones that sell them), he was offering gas pistols, both with $\ce{CO2}$ canisters and springs. The performance of the $\ce{CO2}$ ones was advertized as higher and less handling required, because the gas canister contained more charges than the spring.

What is Carbonic Acid?

Carbonic Acid is what happens if $\ce{CO2}$ is forced into water, or chemically $\ce{H2CO3}$. When the pressure above it is below the maximum pressure, it dissociates $\ce{H2CO3} \to \ce{CO2} + \ce{H2O}$. Yes, it is the stuff that makes soda sparkle.

Dissolved Carbonic Acid

If the capsule is designed in such a way, that a carrier substance holds the gas, and will release it to maintain gas pressure above without allowing the liquid to leave the capsule, then it will act as a gas cannister just like dry cas capsules.

Now, how much gas can you stroe in Water in the shape of Carbonic Acid? Science shows a graph for solubility in Diesel Oil, indicating that it can only store about $\pu{4000 psi}$ in that. The Max Planck Institute did analyze the stabilisation of Carbonic Acid under high pressure and realized that with extreme pressure, you could stabilize it to ludicious temperatures - $\pu{2.5 GPa}$ prevent any dissociation, even at very elevated temoperatures.

While in theory this would allow to store extreme pressure inside a Carbonic Acid, the real limiting factor is the container. And here comes the downside of Carbonic Acid compared to pure gas. It comes from the universal Gas Law $$pV=nRT$$ Pressure times Volume is number of Gas molecules times Gas constant times Temperature.

The carbonic acid is not temperature stable. So increasing the temperature also increases the number of gas molecules. As a result, because the volume and gas constant are fixed, pressure increases much faster as with a dry gas capsule, where the number of molecules is fixed because no new ones are released.

This is compounded by the volume relevant in the carbonic acid capsule is only the part not filled by the carrier liquid - and thus the Volume in the first place is much smaller.

This problem, together with the problem of keeping the actual gun dry, resulted in dissolved gas storge dying out.

Pure Carbonic Acid

You could use pure carbonic acid and circumvent the storage problem to some degree. Pure carbonic acid, when compressed enough, becomes a liquid. We handle that often today: to carbonate beverages, we generally buy liquid, pure Carbonic acid, which then boils off in the upper part of the cylinder, and when released into a liquid carbonates it.

This is pretty much the same as a dry gas capsule, handling-wise, because the carbonic acid instantly turns $\ce{CO2}+\ce{H2O}$. The good side is, that you can store a lot of gas in this shape and you have a very similar performance to dry $\ce{CO2}$.

However, the conditions in the gun won't be stable enough for the Carbonic Acid to stay pure - it will, over time, turn itself into the dissolved carbonic acid type, as some of it dissociates into water, solving some of the gas in it and then breaking down more. After all, the conditions are not the $\pu{2.5 GPa}\ (\pu{3.6 \times 10^6 PSI})$ required to perfectly stabilize it.

Another downside is the production of Water inside the gun as you shoot. Water is the enemy of iron metals and it can result in rust in very hard-to-reach channels, making such a gun hard to maintain. Compared to pressurized dry air (or $\ce{CO2}$), carbonic acid also is more expensive to make and refill.

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  • $\begingroup$ Pure carbonic acid cannot exist at temperatures above −80 °C (−112 °F), unless you somehow manage to eliminate every last molecule of water -- a few molecules are enough to trigger decomposition into carbon dioxide and water. For carbonating beverages we use containers of liquid CO2, not carbonic acid. $\endgroup$
    – AlexP
    Jun 30 at 14:19
  • $\begingroup$ @AlexP there are ways to get almost pure, and as the MPI has shown, extreme pressure can get it done. The main problem with, even if you can get it done, you decompose into water as you release the first shot, even in the canister. $\endgroup$
    – Trish
    Jun 30 at 14:21
  • $\begingroup$ 2.5 GPa is 25,000 atm. Somewhat impractical. $\endgroup$
    – AlexP
    Jun 30 at 14:22
  • $\begingroup$ Yes? read the linked article. It is stable under 25 GPa (3.6 MILLION Psi) and close to 100 °C. $\endgroup$
    – Trish
    Jun 30 at 14:24
  • $\begingroup$ @AlexP but pounded on that a tad for your convenience $\endgroup$
    – Trish
    Jun 30 at 14:36
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As told by @AlexP, carbon dioxide was called carbonic acid.

Different kinds of CO2 Air Rifles are available at https://www.airgundepot.com/co2-air-rifles.html or https://www.ebay.com/b/CO2-Air-Rifles/178888/bn_8250217.

CO2 Powerlet Cartridges for Use with Air Rifles and Air Pistols are available on amazon or ebay. (As told here, a Powerlet cartridge, commonly referred to as a CO2 charger, is a small disposable metal gas cylinder holding 8–12 grams (0.28–0.42 oz) of liquid CO2 and often a small quantity of lubricating oil, used as a pneumatic power source for certain air guns).

Rifle with pellet velocities over 700 FPS in .177 cal and over 600 FPS in a .22 cal is used for pest control or small game hunting.

There are single fire or repeater modes rifles. Some have have a scope or fixed sight to help you get the shot at a distance and with precision.

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TL:DR; Yes there is some difference in performance, but 'Carbonic Acid' powered guns have adequate performance compared to modern air guns.

Given that, as per AlexP's comment, (Anhydrous) Carbonic Acid = CO2, then CO2 is a perfectly valid propellant for 'air' guns. Many different types are available for sale today.

In terms of accuracy CO2 compressed gas guns are as accurate as equivalent designs using compressed air.

In terms of power, the higher pressure available from modern compressors means that, in principle, air powered compressed gas guns are more powerful. In practice while the most powerful rifles today are air powered, many air rifles and CO2 rifles offer similar power, and CO2 rifles are certainly powerful enough for most of the purposes for which airguns are used today.

In terms of cost - well that is slightly more difficult to answer. Mainly due to the huge variety of possible designs for a compressed gas gun. The cheapest air guns are powered by springs, and may be very inexpensive indeed. However, high quality rifles can be very expensive. Generally CO2 guns will be more expensive to buy than budget spring guns, and requiring CO2 in addition to pellets will cost more to shoot.

Air rifles that are filled with compressed air, either directly from a compressor or a bottle similar to a diver's SCUBA tank that is in turn filled from a compressor at a diving or gun shop (or a home compressor if you plan on doing a lot of shooting and need more air than the gun itself can hold) are generally more expensive to buy than CO2 rifles and require expensive accessories to charge them. HOWEVER - while CO2 capsules are cheap enough when bought in bulk, eventually a compressed air rifle will be cheaper to shoot. Exactly where the tipping point is will depend on the local market, but where I live we are talking many thousands of shots before even the cheapest charging solution starts to work out cheaper than CO2.

Other Factors: Generally the biggest draw back of CO2 performance is that it is dependent on the temperature in which the rifle is being used. CO2 generates higher pressures in higher temperatures. This means that if it is used year round in an environment with high seasonal variation in temperature the difference in power between summer and winter can be extreme - possibly around 10% (based on experience, I don't know if anyone has actually published scientific tests). The effects of cold weather are: reduced long range accuracy, shift in point of impact relative to the sights, fewer shots per CO2 capsule.

In my opinion, it is this rather than accuracy or power that has meant that compressed gas rifles powered by air have been more popular in recent decades than CO2, despite their greater purchase cost (if not necessarily Total Cost of Ownership in the long term).

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Presuming that by "carbonic acid" you mean, as did the 18th century writers and gunsmiths, actual compressed carbon dioxide ("carbonic acid gas" was the full name back then), then there are two main differences in performance between a Pre-Compressed Pneumatic gun like a Girandoni or one of the Air Venturi models noted and a bulk-charge liquid CO2 gun.

First, compressed liquid CO2 has a vapor pressure of around 800 psi (~54+ bar), vs. around 3000 psi (200+ bar) common in PCP arms; but second, because the CO2 is stored as a liquid, the pressure remains constant over a much longer shooting life on a charge compared to compressed gaseous air.

The lower pressure of CO2 is compensated by designing the firing valve to simply dose more gas per shot, thus keeping the pressure behind the bullet/pellet supported longer than would be the case with PCP.

On the other hand, PCP can be recharged with a simple pump (in the lowest tech case, a hand operated pump -- takes a long time and lots of work, but no electricity etc. required) by compressing ambient air, while liquid CO2 requires first generating CO2 chemically or separating it from air, then compressing it in isolation; this isn't a very portable process.

So, if you want the best combination of performance and convenience, you'd use CO2, either in so-called soda bulbs ("Powerlets" is a trademarked name owned by Crosman) or in a pressure tank with the correct connectors to charge your gun (or just possibly as dry ice, though this involves a delay while the dry ice warms to ambient in the sealed pressure chamber); but for lowest lifetime cost and (for commercially made guns) highest total performance, you'd usually go with PCP.

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Speed of sound limitations are what limits projectile speed. Gas molecules are moving around the speed of sound in any gas. The bullet can accelerate only if there are enough gas molecules hitting it from behind, so a speed-limit exists. Firearm propellants raise this limit by using strongly exothermic reactions to make extremally high gas temperature with high speed of sounds. However, carbonic acid going to CO2 is endothermic so this will not happen.

CO2 has great storage capacity. CO2 is a heavier gas with a slower speed of sound than air, so why use it? CO2 will liquify at room temperature once it is compressed to ~60 bar (a convenient pressure for an air-gun). At this point, adding more CO2 will not raise the pressure, it will just mean there is more liquid in the cartridge. You can keep going until it is full of liquid. Air doesn't behave this way: it will never liquify no matter the pressure. Capacity is limited unless you have carbon-nanotube tank walls.

Your idea works for hydrogen. Hydrogen also doesn't liquify under pressure. To store hydrogen, they sometimes fill the tanks with a hydrogen absorbent metal and then pump in hydrogen. The hydrogen chemically reacts into a solid form but the reaction is reversed when the hydrogen is taken out. Hydrogen has an extremally high speed of sound at room temperature. You can react half the hydrogen with oxygen/oxidizer and use it to propel a piston which compresses the other half of the hydrogen to a very high temperature. This hot hydrogen is then an excellent propellent to achieve extremally high speeds for a small bullet assuming your universe requires that kind of weapon.

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