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.
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.
@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.