Excuse me if this is somewhat rambling.
I was considering the advanced firearms and I realized that they don't appeal to me. Not only do they not play into my obsessive love of percussion cap weaponry (it just doesn't get enough credit I say), there just seems to be something clumsy about it.
So I was thinking of proposing advanced weaponry simply as a modification of the baseline early firearms, with each only available at certain levels of firearm rarity. They'd be defined primarily by the ammunition they used and each would also represent a general advancement in firearm design and production (resulting in lower misfire chances, greater ranges, and reduced cost). These categories would respectively be percussion cap and metal cartridge.
Percussion cap firearms wouldn't be available in any campaign with a greater rarity than "Commonplace Guns". Percussion cap versions of early firearms would cost 25% less (with early firearms and ammunition still only costing 25% of their original price at this level of rarity), their base range increment doubled, and have their misfire chance reduced by 1; additionally, their reload times would be reduced by 1 step. They also require percussion caps to fire, in addition to a black powder charge and a bullet.
Metal cartridge firearms wouldn't be available in any campaign with a greater rarity than "Guns Everywhere". Metal cartridge versions of early firearms would cost 50% less (with early firearms and ammunition as well as percussion cap firearms and ammunition still only costing 10% of their original price at this level of rarity), their base range increment tripled, and have their misfire chance reduced by 2; additionally, their reload times would be reduced by 2 steps. They require metal cartridges to fire.
Now for some new gear, yay.
Percussion cap: These tiny metal cylinders contain a volatile explosive compound that detonates with a sharp impact. They're either affixed to special nipples on the rear of a chamber or affixed to the rear of a cartridge and are used to provide a spark to ignite black powder. Cost: 10 gp.
Metal Cartridge:: These small metal casings combine bullet, percussion cap, and powder into one unit. Making for substantially easier loading and storing of firearms designed to make use of them. Cost: 10 gp.
That last bit was just included as what I consider a more suitable explanation for this system and to bring cartridge prices within the standard of ~10 gp per shot for 'cutting edge' firearms.
Edit: I realized with this that some slight modification of the alchemical cartridges was in order. So here we go.
Alchemical Bullets: Alchemical bullets are specialty rounds for firearms that come in a number of different varieties each with their own effects.
(copypaste the rules for alchemical cartridges here, excluding paper cartridges, remove all references to increasing misfire chance and reduce all prices by 10 gp. Reduce Salt Shot to 1 gp. Use the same base prices for metal cartridge ammunitions; save Salt Shot which should only cost 10 gp).
Paper Cartridge: This is a simple paper wrapping surrounding a combination of bullet and powder. Increase the misfire chance of any firearm using paper cartridges by 1 and reduce reload times by 1 step. Cost: 1 gp plus the cost of the powder and bullet.
I would also include options for the metal cartridge are they using regular gunpowder or smokeless as there is a major difference in range and stopping power when used.
I would also include options for the metal cartridge are they using regular gunpowder or smokeless as there is a major difference in range and stopping power when used.
I figured that would be a bit advanced for the entire scheme. But it wouldn't be too difficult just to add another category of weapon to the mix and rule that black powder weapons suffer an increased misfire change using smokeless powder cartridges.