For the game "Faded Haven" by Pheonix Dreams Entertainment.
Basically, a form of 'submachinegun' or 'assault rifle' using the classical but other-worldly technology of the game's universe.
The bullets are ordinary musket balls, but are poured into the lidded box at the top (which in turn tumble into the firing chamber).
Propulsion uses a material that regenerates a supply of explosive fluid that can be ignited by match or flintlock (the latter applies for most of these guns).
However, normally this process takes a moment, a rapid-fire gun normally wouldn't be possible! So, to accelerate the rate of fire, a revolving mechanism containing a number of these propulsion cores had been added- the mechanism spins rapidly as the gun fires, giving the mechanisms more time to regenerate.
To retain a classical appearance, and imply advanced but unusual technology, I based the gun partly off the Chu Ko Nu (the Chinese repeating-crossbow), and mixed it with the bulky metalic revolving core (the spinning being reminiscent of classical gatling guns!)
The rear mechanism is rather simple as I couldn't figure out what to do with it- and rather than hold back the project just for that, I decided to leave it as-is for now, as I wanted to see if the overall appearance was in the right direction!
Hope you like it!