Keeping single barreled guns relevant is just one of the reasons to nerf double barreled guns a bit. In case it wasn't clear, my proposal to limit the "double shot" to a standard action is separate from my proposal to slow down reloading but offer a full attack alternative which works more like Mythic Vital Strike or Pummeling Style (which are actually two pretty powerful options IMO)
The standard action for double shots is the sort of thing I could imagine Paizo actually coming out with in a FAQ even though it apparently wasn't the original RAI. Giving Gunslingers an effective full attack alternative in place of the ineffective current version of Dead Shot is pretty much just me daydreaming since that would be a pretty significant change.
Well, they can also kill things without alerting the entire enemy base to their location with the loud explosion their weapon makes each time they use it, which is useful for every stealth-based character that has ever been rolled, but eh.
This is not a problem exclusive to gunslingers. Every non-stealth based character makes a lot of noise
Oh, also your @#$&awesome touch AC attacks will run into situations where you're not ahead of the others at all. Such as literally any time you encounter an incorporeal undead.
Yup in that exact, specific, unique, rarely occurring situation, gunslingers are only slightly better than archers since the gunslinger is all DEX and the archer is spread over DEX and STR.
Most Archers figure out how to ignore concealment more easily than gunslingers, too.
Just sayin'.
Do you know why archers figure it out better? Gunslingers don't have to. Add your +4 to concealment, when the touch ac is 13 lower than regular ac, you still net a +9 over the archer.
And yes everyone carries a ranged weapon and it's usually a sling or crossbow, both with the same range increment as a gun.
You'd be getting something significantly better than Vital Strike without even spending a feat, so I don't think being unable to combine it with Vital Strike would be a big deal. I say it is better since you'd be able to apply your Dex bonus, Deadly Aim, and other modifiers twice whereas VS only gives you base damage.
If there's a problem here I think it is really with Vital Strike, which already doesn't work with almost anything else.