Seriously.. take a level in a major casting class, and grab this first level spell:
Abundant Ammunition
School conjuration (summoning);
Level bard 1, cleric 1, ranger 1, sorcerer/wizard 1
Casting Time 1 standard action
Components V, S, M/DF (a single piece of ammunition)
Target one container touched
Duration 1 minute/level
Saving Throw none;
Spell Resistance no
When cast on a container such as a quiver or a pouch that contains nonmagical ammunition or shuriken (including masterwork ammunition or shuriken), at the start of each round this spell replaces any ammunition taken from the container the round before. The ammunition taken from the container the round before vanishes. If, after casting this spell, you cast a spell that enhances projectiles, such as
align weapon or
greater magic weapon, on the same container, all projectiles this spell conjures are affected by that spell.
1 minute per level means it'll last most fights. Spring for the more costly paper charges (have a couple just in case), and then never actually "use them up" as long as you can cast this spell first.
Each combat should start with the following round: cast spell on ammo container, move action to load weapon. From here out, you can attack at least once a round.
This should work for the first few levels at least. Extremely helpful with the cost of ammunition for firearms.
Also good if you need silver bullets, or adamantine bullets, and you want to shoot them all willy-nilly without losing cash like crazy.
I'd suggest grabbing a bard level, so you can cast while in light armor. Bards can cast without penalty while using a shield too, and there's no penalty to using firearms with a buckler on.
If you can get Rapid Reload along with the paper charges, you'll be able to reload as a free action (I think? that might be still up for debate), making a double pistol and eventually a pepperbox a valid weapon for iterative attacks.
*Edit* In retrospect, these won't even be necessary. If you reload as a free action, you can reload the same barrel over and over.
Dual wielding will be tough. You do need a free hand to reload the weapon, even if you can do this with free actions. Weapon cords allow retrieving one weapon as a swift action, but you can't drop both.
Eventually, if you can get your hands on a glove of storing, you might be able to pull it off with a weapon cord combination: drop one, reload and fire, snap fingers to store, swift to retrieve and reload and fire? I guess it depends on how you have to run your iteratives... a single offhand shot from TWF would work, but if you have to intermix your iteratives on primary and offhand, then ITWF and GTWF might not be possible).
*Note* In normal games, you can rely on a teammate to pick up this spell and have them cast it on you to keep you effective (at low cost), but then you could also rely on treasure splits, and if we want to go into it, rely on being able to craft things (does PFS at least allow you to craft your on bullets?).
In PFS, you kind of have to be self reliant on something so important to your class ability. Picking up a single level of Bard could solve a lot of problems.
If you don't like the idea of a "Bard" level... go with the
Archaeologist archetype: no performances, you have a Luck ability instead (activates on swift action at 1st level, and starts off with your max rounds, it doesn't scale.. pick up Lingering Performance and triple your rounds of use). He's basically a rogue that has some bard magic talent (gaining your abundant ammunition spell).
It opens up access to Arcane Strike too, to gain magic and bonus damage on your shots (on rounds you aren't using your swift action for other things, like the two lingering rounds of luck if you grabbed that feat).
Good luck with that 
