Any black powder firearms in Vintage Story would have to be balanced in a way that fits the gameplay design, and that means *attention to detail in the mechanics of how they operate.*
You would never carry around a black powder firearm already loaded, for risk of it fouling or going off unintentionally. This means it wouldn't be practical to do the pirate cliche of carrying a bandolier of loaded pistols.
Black powder firearm mechanisms are not even remotely reliable. A badly made black powder firearm is as much a danger to your enemy as yourself.
To give an example of how this would look in the game, let's use a matchlock, one of the earliest forms of a proper musket as an example.
Matchlocks should be creatable out of Bronze, Iron, or Steel. It should however be gated behind steel tech. Probably a special type of tool that would be used to forge the fine-scale springs and mechanical parts to make it work. When you have created your matchlock musket. The gun would take paper cartridges (crafted with beeswax, lead bullets made with a mold, blasting powder, and of course paper).
When loaded, the gun starts a countdown timer, depending on the metal used to create the gun, and durability, that is no longer than 10 minutes, but for a low durability bronze musket, should be quite short. The timer is reduced by 8 minutes if you enter water. and the wick also would go out and need to be relit by crafting the gun with a torch. After the timer expires, the gun is fouled and will not fire until it is reloaded.
Furthermore, when a gun's durability is low enough that it would break on its next use, it instead will explode in a shower of disorienting sparks and do a fair bit of damage to the user, as well as generate a fair bit of smoke. Furthermore, smoothbore muskets are not the most accurate tools. More accurate than portrayed in a lot of media, definitely not as accurate as a bow, so you may as well just miss.
However, when you do successfully fire the gun by taking the necessary precautions and time spent to load it properly, you are rewarded with likely the single most damaging attack type in the game, likely capable of killing or severely injuring a player through even steel body armor. and all of this locked behind realism, a considerable time investment and game mechanics that create an environment where managing to actually shoot the gun is a reward in of itself. In this way, I think black powder firearms could be made a very fitting and enjoyable part of Vintage Story.
