Jolly Ruby
Privateer
I like the idea, but a lot of mechanics related to combat would need a gargantuan overhaul. Like it happened in real work, once you introduce modern firearms the whole system that existed before falls apart. The closest thing we have to firearms in D&D is the spell disentegrate. Dexterity saving throw and if you fail you die (or take up to a hundred damage). Once everyone have modern firearms melee is not an option anymore, armors are useless, cover is a lot more important and the wizards would have to develop new spells to survive (mage armor and shield are useless now).