One of my long-running Gamemasters used firearms in his games along with black powder and he did an excellent job of it. Firearms were exotic and cool -- and the powder itself was pretty dangerous.
Anyway, for this new edition of D&D, I wanted to write up a gunfighter or pistoleer or musketeer, or whatever you want to call it, so I did.
Personally, I think this archetype has one or two too many abilities, but in order to make it work the way I wanted, this is what I ended up with. It's also entirely possible I haven't thought through every rules problem well enough. While I like the Coolheaded abilities, adding that d4 to a cleric's bless d4 or a bard's bardic inspiration d4 or d6 might be a bit much. But then again, bless and bardic inspiration exist next to each other already, so maybe it's not so unbalanced. Some of the abilities are also limited so they only apply to firearms, so hopefully that balances the abilities a bit. (I made some edits so the abilities come at the correct levels.)
I should add, this archetype requires a new background to give it proficiency with gunsmithing tools and access to a pistol or musket, which is priced out of reach for beginning characters. I might write up that background at some point.
Anyway, enough talk! Check out my take on a gunfighter for D&D:
Black Powder Irregular
Some warriors work for lords who have mastered, or have themselves otherwise learned, the secret of black powder. These gunfighters are trained to keep their cool and make the most of every shot. This archetype uses only the Renaissance Items off the Firearms table (see the DMG, page 268).
Beginning at 3rd level, while you are wearing no armor and not wielding a shield, your AC equals 10 + your Dexterity modifier + your Wisdom modifier. If you are wearing light or medium armor and not wielding a shield, you gain a +1 AC bonus.
Beginning at 3rd level, you gain advantage on initiative rolls, and, if you aren’t within 5 feet of an enemy when you attack, don’t move more than 5 feet during your turn, you may add 1d4 to your first attack roll with a pistol or musket.
At 7th level, your pistol and musket ranged attacks score a critical hit on a roll of 19 or 20.
Additional Fighting Style
At 10th level, you can choose a second option from the Fight Style class feature.
At 15th level, you may move up to 15 feet during your turn and still gain the bonus to your attack rolls as long as you’re not within 5 feet of an enemy when you attack, and may add 1d6 to two attack rolls with pistols or muskets instead of 1d4.
Also starting at 15th level, your pistol and musket ranged attacks score a critical hit on a roll of 18–20.
Starting at 18th level, once per turn, when you hit a creature with an attack using a pistol or musket, you can wound the target. At the start of each of the wounded creature’s turns, it takes 1d4 damage for each time you’ve wounded it, and it can then make a DC 15 Constitution saving throw, ending the effect of all such wounds on itself on a success. Alternatively, the wounded creature, or a creature within 5 feet of it, can use an action to make a DC 15 Wisdom (Medicine) check, ending the effect of such wounds on it on a success.