I made this build like a year and a half ago, but didn't post it. It may be pretty normal, but I certainly like it's edge and flavor.
Human: Fighter, Ninja: Scout archetype, Gunslinger: Maverick/Mysterious Stranger archetype
1. Fighter: TWF, Enforcer, Bludgeoner
2. Nin/Scout: Sneak 1d6
3. Mav/Myst: Weapon Focus: Pistol
4. Nin/Scout: Vanishing Trick
5. Mav/Myst: Quick Draw
6. Mav/Myst:
7. Nin/Scout: Shattered Defenses, Sneak 2d6
8. Fighter: Weapon Finesse
9. Nin/Scout: Improved Critical, Combat Trick: Improved TWF
10. Nin/Scout: Sneak 3d6
11. Nin/Scout: Critical Focus, Violent Display
12. Nin/Scout: Sneak 4d6
13. Nin/Scout: Disheartening Display, Pressure Points
14. Nin/Scout: Sneak 5d6
15. Nin/Scout: Invisible Blade, Assassinate
16. Mav/Mist: Extra Ninja Trick: Crippling Strike
17. Fighter: Dastardly Finish
18. Fighter: Blinding Critical
19. Fighter: Staggering Critical
20. Fighter: Stunning Critical
Dexterity first, Charisma second.
Enforcer & Bludgeoner with Shattered Defenses, used with gun pellets.
Quick Draw for Pistols. Class Gunsmithing feat to make 12 pistols with appropriate gold.
Weapon Finesse and all Critical feats with Wakizashi.
And the rest is self explanatory. I suggest you make no changes to level and class order.
-----
I was inspired by the Django Unchained movie gun fight scene, and a lot of gun popping rap music.
In my mind I thought of this masked and robed marauder looking dude with like 12 pistols strapped all over his body, and he just enters a saloon and just unloads all of his guns on everyone, and drops the empty guns (hopefully teleporting back on his holsters somehow). Once out of ammo, he'd rush in his wakizashi's and make a bloodbath of critical hits.