(I did some forum searching (revolver, stephen king) prior to making this post so as not to post something that's already been discussed)
While I haven't played the gunslinger yet, some of the reviews and discussions have a few similar complaints. Not enough damage, revolvers would be necessary/overpowered, other classes abusing firearms, and the like. Having re-read Stephen King's magnum opus of late, one of the things that Roland (the protagonist) does is maintenance on his guns, including the use of gun oils. Now, without wishing to rewrite Golarion to some degree, perhaps that might make things easier.
Flintlock pistols are not terribly mechanically difficult to make, nor hard to use. When I was young, I asked why people used guns if bows, by comparison, fired faster and more accurately. One of several reasons, was that firearms were easy to use. To use a bow, one required some measure of practice to build muscle, learn to aim, and so on and so forth. Firearms require minimal training by comparison. He went on to say that even a man injured in the field of battle could use a firearm without too much difficulty. Why do I mention this? I don't believe it would be too much to make flintlock pistols common technology, however expensive by comparison. Making flintlock pistols simple weapons would be my next step, but not exotic by any means since it often refers to some amount of special training required to use with proficiency.
If we assume that flintlock pistols are expensive but more commonplace, Revolvers can become the weapon of choice for the gunslinger. Revolvers (or if you prefer, six-shooters) are mechanically more complex. This makes them more expensive but more prone to wear and tear, one of the reason Roland often cleaned his guns. In his world, many had never even seen such a thing. In truth, the technology to make them was lost. Along the same lines of thinking, I think of how the Colt M16 was falsely advertised as self-cleaning and not issued with cleaning kits. A gun enthusiast friend of mine has had opportunities to use M16s from the Vietnam Era, which to this day need to be cleaned quite regularly because they were not cleaned during the course of duty.
With that in mind, Paizo can add rules concerning revolvers needing regular maintenance so that they don't jam. Without the PDF in front of me, I can't be certain, but one of the 1st level abilities is called "Firearms". One or two added paragraphs about gun maintenance would likely be enough to describe that a gunslinger is trained to know how to extract oils from certain types of flora that allows a revolver to be cleaned of residue that would eventually lead to a revolver to become unusable. This should allow classes other than the gunslinger to use firearms with a limited rate-of-fire at lower levels. At higher levels, the cost of a revolver would be negligible compared to income, meaning that other classes could use them until they were jammed and have multiple to compensate for the problem, much like the use of a Wand of Scorching Ray.
Well, that's my thinking anyways.