Spazzed on noticing the inability to cover iterative attacks.
Simple method assuming you're sticking with the 'one shot' firearm: Starting 5th level instead of getting extra attacks, you add your wisdom modifier to damage rolls each time you'd gain an additional attack. Or consider something similar to the Paladin's Smite ability. Keeps the Gun fighter dishing damage at a good rate mechanically, and tying it to Wisdom gives the flavorful example of "the gunslinger with the better instinct can cause more damage."
