Some GMs might now that problem:
You would really love to give your firearm proficient character a revolver or other advanced firearm at some point, but you hesitate.
Handing out an advanced firearm makes it difficult, not to include some kind of twist into any major encounter, that is tailored around the question: “What does this enemy do, not to get hit by bullets?”. The reason for this is, that advanced firearm hits 'on touch' within it's first five range increments.
A GM will inevitably encounter times, when revolver-wielding character with a high or even medium BAB progression only rolls his attacks to see, if he either made a critical threat or if he bodged/misfired.
Although I had in my time a lot of ideas and twists up my sleeve (a magnetic pull that made impossible to shoot anything further than 30 feet away, War-Golems that the gunslinger couldn't attack on touch, because their armor was made bullet-proof, defensive training for firearms and bullet shield armor, ninjas that could dodge bullets with Snake Style, aso.) many encounters aren't much of a challenge for a gunslinger/firearm proficient character. It is either hit or misfire, the player even told me that it is 'too easy' for him.
I could continue with this, putting spells on him, enemies disarming him or hiding in cover... but I think this would get frustrating, if I do that in nearly ever encounter.
So my idea is to change the way firearms attack (picked that up from another post):
Instead of attacking on touch as described, a firearm lowers the AC of an target by -4, but the AC can't fall below the touch AC of that target. The reason is that a bullet is able to penetrate armor but does not completely negate it, you can still a spot where the armor swallows the entire impact.
Example: The bullet penetrates a shield (shield bonus), is slowed down even more by the armor (armor bonus) but then doesn't penetrate the thick hide of the creature (natural armor). In other words: The Gunslinger tried to hit with a +21 but, the giant in front of him has an AC of 26.
So, if you shoot someone whose combined natural, shield and regular armor bonus is +3 AC, you only lower his AC by 3, hence the enemy dosen't fall below touch AC.
Of course, after the first range increment (early firearms) or fifth (advanced) the attack is resolved normally.
With this mechanic you can even introduce new firearm specific enchantments and bullets.
Piercing Enchantment:
+1 Enchantment
You lower the AC of an target for an additional -2. The AC of your target still can't be lower than his touch AC. This enchantment does not stack.
Piercing, Alchemical Charges/Bullets:
(Same as the enchantment, 50 bullets)
Feat, Finding the weak spot
Prerequisites, Weapon Focus (Firearms), BAB +6
You are proficient in finding the weak and soft spots of your enemies. While using a firearm you lower the AC of your enemy for an additional -2. This stacks with magical enchantments. The AC of your target still can't be lower than his touch AC.
( Add: You could also handle this feat like Deadly Aim / Power Attack. )