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. 347
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 <PERSON> 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. 537