GUNSLINGER MARTIAL ARCHETYPE for the Fighter
Gunhawk (3rd level)
You are a master of modern firearms. You gain proficiency with all modern martial range weapons and Gunsmithing tools.
In addition, if you may use a modern automatic pistol or revolver in your off-hand and gain your Dexterity modifier to the damage roll.
Finally, being within 5 feet of a hostile creature doesn’t impose disadvantage on your ranged attack rolls with modern firearms.
Rain of Bullets (7th level)
You are a haze of bullets. When you use your Action Surge, you gain advantage with your ranged attack rolls with firearms.
Sharp Shooting (10th level)
You are a crack shot. You gain a +2 bonus to damage rolls with firearms.
Bullseye (14th level)
You are an angel of death. When you hit a creature with a range attack with a firearm you deal an extra 3d8 damage. Once you use this ability, you cannot use it again until you finish a short or long rest.
Lightning Shot (18th level)
You are the fastest of the fast. If you do not take a move action, you may use your reaction to make a ranged attack with a firearm.