How to make pneumatic weaponry/airguns the main weapons of a miliary force?

Question

Perhaps you know of pneumatic weapons also called airguns, but for those who don't; they're weapons that use air as propellant to launch a projectile rather than for example black powder or smokeless powder. They've been used as battlefield weapons, the most notable is probably the Girardoni air rifle https://en.wikipedia.org/wiki/Girardoni_air_rifle , but others such as the Kunitomo air gun https://en.wikipedia.org/wiki/Kunitomo_air_gun also existed.

They had some advantages and some disadvantages in that period of time, but eventually the disadvantages won out over the advantages. Air gun tech has come a long way though, so their capabilities have increased, of course so has their concurrents (such as smokeless powder) their ability.

My question is as follows. How could weaponry that uses air as a propellant become the norm for a military force? What kind of limitations (environmental/logistical/industrial/scientifical/economical), preferably no theological would be required to make a society favor fielding pneumatic weaponry over conventional weaponry?

I can only think of a subterrenean setting due to the noise reduction and shorter ranges involved, however I would like to also keep this above ground the norm. Not only that I would like stuff like Man-portable anti-tank systems (though stuff like PIATs and panzerfausts would also be in use), mortars and artillery to also use air as propellant.

Airzooka

Things to keep in mind:

-The setting in which this is to be used is semi-(post) apocalyptical, so mass production is not really an option; cottage industry exists and people would be familiar with modern techniques (though for obvious reasons stuff that requires computers and such is not feasible).

-People know about smokeless powder, black powder,...

-There are monsters in this world, so maybe they can play a role, but I feel like they shouldn't be the only reason.

Edit:

@TheDyingOfLight That's not a bad thinking route you're using there.

@KerrAvon2055 Biggest armies (not including coalitions or alliances) are regiment sized, biggest battle will be fought at battalion size and most combat (90%) squad to platoon size. Though most of these people will not be professional soldiers, but conscripted/levies. Reasons for fighting vary, most of it boils down to I want what you have.

As for the monsters; varies greatly. There are things that are not any or barely sturdier than humans to things that could use a bazooka or 50 call. to bring down and then there are those where you better use flamethrowers or more specialised stuff (like magic (haven't worked that out yet) or certain elixers).

The terrain featured shall be urban, forest, swamp, subterennean and something like the Scottisch highlands.

@JBH I really enjoyed your comments. As you said silence is a bit of a problematic advantage since the compressors are quite the opposite.

@GrumpyYoungMan Yeah, I know that pneumatic can't match conventional hence why I am trying to find ways why people who know of smokeless powder don't use it in favor of air. On the other hand if everybody is stuck with pneumatic then everyone suffers from the same issues. The only way something like the long Tom could be fired seems to me if they're using it as a stationary emplacement with dedicated facilities something like the V3 was meant to operate in WW2. That said I am probably going to allow them to have crude rockets whether they're powered by blackpowder or something else is still something I am thinking about.

@Duncan Drake The location is not really based on any place on Earth really, it could be, but it's not like I am ever going to say "Northern America" or something like that. In fact I am probably going to make the place a sort of alternate dimension and hint at that. I just don't want to do stuff like in this dimension you can't make gunpowder work, because I feel like that's a bit of a cop out. One thing though that may be important is that the place is often very foggy or at least misty. Maybe that helps a bit to off set the lower range of pneumatic rifles. So yeah, no ammunition imports.

Answer 1

No Dangerous Chemistry Required

In a post-apocalyptic setting that brings the society back down to cottage-scale industry, gunpowder becomes a finite resource. The chemistry involved in making safe, reliable smokeless powders is complicated and involves noxious, caustic, or volatile precursors, as well as processes whose failure modes involve very loud explosions. Many early nitroglycerin and nitrocellulose factories were completely destroyed due to accidents. Even black powder manufacture requires chemical precursors which could be hard to find or manufacture in the required concentrations and purity in a post-apocalyptic setting.

Safe production of an airgun, however, is well within reach of a reasonably-equipped machine shop. Compressors would be available via salvage or new production, and pressure-rated bottles can be found everywhere, in sizes ranging from CO₂ cartridges to SCUBA or welding-gas tanks (or even larger).

Answer 2

First, what are the advantages of air guns?

• Good for stealth. The Wikipedia Article on air guns mentions that "they did not disclose the shooter's position or obscure the shooter's view, unlike the black powder muskets of the 18th and 19th centuries." Probably similar with-gunpowder weapons could be made, but I'm not a gun expert. Wikipedia also notes that they could be deadly in the hands of a sniper because of their quiet nature. It says that "France, Austria and other nations had special sniper detachments using air rifles."
• Cost-efficient. There's no use of gunpowder, so that's a bit saved on every shot. It probably wouldn't be much in our world, but maybe in yours the resources to make gunpowder are more rare or difficult to make. (or, as you mentioned, mass production is not an option which might make gunpowder more costly)
• Versatile. Air guns can be used in all kinds of different environments. Guns with black powder, for example, have issues with getting wet, and there are probably similar restrictions for other powder types (as I said, I'm not an expert). Your world could have a lot of environments that make gunpowder an issue where air guns wouldn't be (you mentioned swamps as a battleground, which would be a problem for black powder because of the water). My favorite example is what I'll go over next.

I have to mention what I think is possibly my favorite gun design I've read about: the air rifles in Twenty Thousand Leagues Under the Sea. These guns are designed to be used for hunting underwater which I think is totally awesome. The reason Nemo chooses this type of gun is that he cannot manufacture powder from his submarine (as I said, this is resource-effective), but my favorite part is that he makes up for the slow speeds they'll have so far underwater by electrizing the balls. If one of them so much as brushes against the target, it is lethal and the animal drops dead from an electric shock. Now, this might be possible with gunpowder weapons, but this is the only example of the idea I've seen. (You could also replace the electricity with some sort of magic thing, I don't know how your magic system works)

I hope this gives a few ideas. If these don't work, let me know and I'm happy to come up with something new.

Answer 3

Market dominance, tunnels and sniffer monsters

There are three reasons I can think of for air-powered small arms and low-velocity support weapons to be prevalent in the environment described. (Sorry, but @GrumpyYoungMan is correct that the technology is not feasible for serious artillery. Cottage industry would also be struggling to build 155 mm guns and their ammunition.)

First point - the best technology is not always adopted. The person with the best marketing typically wins, especially in a "cottage industry" level where there are not going to be competing corporations with professional marketing teams. All it takes is for there to be a bad experience with the only competent chemist blowing their hand off while trying to manufacture primers while a highly competent air gun enthusiast gets a production line going and air guns will be first choice for one enclave at least. (Hard to apply the same set of circumstances to all the nations, but if one enclave starts selling the air gun equivalent of the AK-47 to everyone then it will become dominant.) The ammunition will be very simple to manufacture compared to projectile + cartridge case + powder + primer, with no one in a position to contest the manufacturer's claims that the increased maintenance on the air gun seals is less than the increased cost of gunpowder ammunition.

Second - air guns are much safer to use in poorly ventilated areas. If a significant amount of the fighting is occurring in tunnels, basements or small rooms, the ability to not asphyxiate yourself while shooting is a major advantage. I realise that most of the terrain mentioned was open ground fighting, but if useful technology / resources are in underground caches and basements then there may be fighting going on there. (The reduced peak decibel level of the airguns is also beneficial, as noted by Benjamin.)

Finally - one idea that was proposed several decades ago was to create venomous insects that would home in on the odour of the brand of gun oil used by enemy forces. This was a silly idea - it would take much longer to design and breed the insects than it would take for the enemy to change brand / aroma of gun oil once they learned of the threat. However, what if one of the pre-apocalypse factions was practising asymmetric warfare and had no conventional forces at all? Create a bunch of venomous insects (let's say wasps) that home in on all gunpowder combustion products and suddenly having lots of gunpowder guns does not look like such a huge advantage. Assume that these insects have become ubiquitous in the relevant area since the apocalypse.