So, I've had this idea which doesn't fit in my world (I've only got humans), but I thought it was neat so here it is: Dwarves would invent bullpup firearms and here is why.

With early muzzleloading firearms, you have to reach the muzzle of the gun to be able to load it, which poses a problem for dwarves since if your gun is much taller than you it would be difficult to reach. You can't just shorten the gun, since with black powder you need a long barrel to get it to fully burn and give your ball enough power. Furthermore, a long gun would also be unwieldy inside small dwarven corridors, which would be extra important since with dwarves living underground they'd be more likely to be fighting indoors.

The solution to this is to bullpup their muskets. I'm fairly certain that there aren't any significant technological barriers to making it work, there just wasn't much reason to do so in our world. This would let them load their guns normally, without sacrificing power, and make them much easier to carry around and use indoors.

The main issues with this are that having the flashpan so close to the user's face would burn them, and the hearing loss. The flash is dealt with by that plate that sticks out beside the lock, which would hopefully deflect the explosion away from the user. The hearing loss I think could be compensated by the fact that the dwarves, having to fight indoors underground oftern, would probably have much better hearing protection much earlier than in our world.