So, somthing that I became interested from the firearm topic was air guns. They operated on compressed air and originated in the 1700s (at least as far as I know).
The enlarged buttstock is actually an air tank. It used an elaborate system contained in this back pack to pump the gun up again.
Here is a later version from around world war II.

Ok, here are a couple I drew that I think could fit into Kenshi's asthetic.

The one on the left features a bicycle style pump that doubles as a shoulder stock, and an old-timey crossbow style trigger. It would have to be placed barrel down to pump up. The one on the right, has a more conventional trigger and a well pump style pumping device. This device is patented by me, and I dub it 'Sir hand eater'. Both are breech loaded and could use darts, or balls or just about anything.
I can just imagine an imperial conscript frantically trying to use that bicycle pump.





