In the Future?
For my own Shattered World setting, I have kinda pencilled in the idea that there are a few places where the gun has started to make a (re)appearance, using special alchemical 'firepowder'. I only intend for them to be basic hand cannons at this stage, though there's scope for better if Pcs want to approach the question.
I say reappearance advisedly; the setting involves a major Cataclysm that destroyed the world (literally), so there's also scope for finding a few leftovers from the major wars that destroyed civilization. What form these will take, I really haven't decided.
I am afraid of the 'Barrier Peaks' type absurdity if I handle it badly. Anyone else try and include a few firearms of more advanced design?