Two people here have an fascination with firearms. I down own any firearm, but i visit firing ranges often, so an small comment mentioning balance and using some IRL examples of how balance is mostly in conflict with variety, immersion and historically accuracy lead to this. Anyway, introducing firearms into a D&D game can be interesting. For example, would mages try lobby to prohibit it? And assassinate those who are developing this new research to avoid loss their monopoly on mass destruction? And firearms would be that amazing in a world with magic? If an barbarian tribe can use spells, an situation like Pizarro destroying an civilization would be possible? How aristocrats would see this new technology?
There are firearms and artillery in the Forgotten Realms, but the gods have conspired to make them unreliable. There's also a campaign called Maztica
based on the Spanish conquest of the new world. Amn (IIRC, it's been a while since I looked at it) takes the place of Spain and the church of Helm plays the part of the Catholic church.