This is not a Gun stunt but an option I allowed in my Campaign was a 1-time offer to spend a Fate Point to get some extra Skill Points. I figured I'd put it here as an additional "stunt" of Sorts.
[-1] Extremely Skilled; +5 Skill Points
That seems
highly overpowered to me. Most stunts confer 2 shift bonuses, or similar, many with various conditions that have to be met.
[-1] Two-Gun Mojo; can make a Second attack with an off-hand weapon as a Supplemental Action with the penalty applying to both attacks. An additional Supplemental move action can be taken but it will impose an additional -1 to all actions
Again, I'd say this seems a tad overpowered. I'd create it more like the two-weapon fighting stunt for weapons; adding half the base rating of your second gun to your overall weapons rating.
It'd look more like:
[-1] Double Tap; May add half of the base Gun damage against one target on top of the regular damage
Just subtly different, with a different flavour. This stun seems much more balanced.
[-1] *Gun Kata; may use Guns instead of Athletics for Defensive Tests against Physical Damage attacks (Fist, Weapon, Gun or Magic) but not Social or Mental attacks
Potentially balanced, but you might want to add a more specific condition. Although not necessary, its borderline-balanced already, but an added condition might just ensure that fact; or perhaps I'm simply worrying too much.
Just my thoughts. Interesting concepts for the stunts though.