Only one I'm aware of (besides the Gunslinger advanced class in D20 Modern), given my limited spending money, is my own prestige class the Gilden Fusilier, on my Rhunaria homebrew's page (as linked in my sig; the PrC is in the PrC section and the firearm rules are in the miscellaneous houserules section).
Not sure it'd fit what you're looking for, and it's definitely a bit strong as a PrC (but strongly limited by roleplaying matters in my setting; flintlock guns are new and rare, and the PrC requires involvement with the King's military and thus a certain degree of acceptance from the Royal Magi, if the Fusilier ever intends to acquire magical guns or magical bullets). Heavily gun-focused though; no real benefit from the class in melee, d6 hit dice, mediocre skills, and effectively neutered when lacking a loaded gun (and Rhunarian guns use some realistic rules for loading and firing; no Rapid Reload feat application, cannot be made masterwork (though magic enhancements are doable regardless), can't be fired more than once every 1-2 rounds, small chance of failing to fire (due to minor loading mistakes, more likely if nonproficient), takes longer to load if nonproficient, much more expensive and rare than most other mundane weapons, etc.).
Basically though, the PrC gains a few bonus feats for firearms use and shooting on the move, including Weapon Focus and Specialization in flintlock firearms at 4th-level, along with class features that aid in sniping, moving among treetops and other good sniping positions, reloading on the move, firing two guns at once, and imbuing firearms or bullets with minor, temporary enhancements once in a while.
I can't afford Ptolus, so I'm not sure what rules Monte Cook made for firearms in the setting, and thus I'm unsure if the Gilden Fusilier would be stronger with such firearm rules (as-is, I know it's balanced sufficiently for my Rhunaria setting, where wealth is less abundant than in the core, and adventurers aren't walking vaults).
