I actually think you're going the wrong way to declare them Advanced or even Martial weapons. It might be better to showcase the firearm's most historically advantageous trait: just about any peasant can use them with a few weeks of training.
With this in mind, all early firearms are Uncommon Simple weapons:
-Pistol- 1d8P - 20 ft range - Reload 1 - 1 hand - Spread, Recoil 1, Deadly 1d10
-Musket- 1d10P - 40 ft range - Reload 1 - 2 hands - Spread, Recoil 2, Deadly 1d12
-Spread- The weapon is inherently inaccurate beyond a certain range. When Striking a target beyond the first range increment, you miss unless you would critically hit, in which chase you hit instead.
-Recoil- The weapon has a kick that throws off your aim for future shots. When you Strike with this weapon, increase the multiple attack penalty by the listed number for any further attacks. Reduce this rating by your Constitution Modifier, and by a further 1 if you are wielding a 1-handed weapon in two hands.
In this case I'm assuming the prevalent use of alchemical cartridges as proposed earlier in this thread, which combined with some suspension of disbelief allows the Reload of 1 for both weapons which historically (both IRL and in PF1) took much longer.
The assumption of alchemical cartridges also allows for some additional versatility, as presented by the ammunition types or variant weapons below (I'd recommend you don't use both).
-Ball Cartridge (default)- No modifications.
-Shot Cartridge (packaged shot for shredding damage)- Double the Recoil by 1, replace Deadly with Fatal.
-Blast Cartridge (dragons-breath rounds)- reduce the range by half, add the Blast property.
-Spike Cartridge (Flechettes)- Double the Recoil, add the Penetrate property.
Pistol Variants: Dragon Pistol (with Blast Cartridges), Hand Cannon (with Shot Cartridges), Bolter (with Spike Cartridges)
Musket Variants: Blunderbuss (with Shot Cartridges), Dragon Musket (with Blast Cartridges), Heavy Bolter (with Spike Cartridges)
With the properties as follows:
-Blast- The weapon creates a conical blast of length equal to weapon range when you Strike, dealing weapon damage as normal to all creatures that would be hit in the area. Attack actions you take (including Strike) require an additional Action and attack rolls are made as though you had made an additional attack with this weapon (for the purpose of multiple attack penalties).
-Penetrate- The weapon creates a linear blast of length equal to weapon range when you Strike, dealing weapon damage as normal to all creatures that would be hit in the area. Attack actions you take (including Strike) require an additional Action and attack rolls are made as though you had made an additional attack with this weapon (for the purpose of multiple attack penalties).
All the other canonical Pathfinder guns (like the Buckler Gun) can be easily created from there by combining properties above with those in the core rulebook (add the Attached property to the Pistol, while reducing the damage die). For extra spice, make different guns or ammo types different levels, add Advanced firearms (at Rare) by reducing the downsides and having a larger clip size, or create new ammo types (Blast Cartridges with Fire damage anyone?).
What are your thought?