Are there any supplements that expand upon firearms? Like new rules and creation options? I'm interested in if there are rules for rifle-muskets firing mine balls. In the base game there are only rifles that fire metallic cartridges and smoothbore muzzleloaders.
Most of the rules for normal firearms are found in Ultimate Combat. I'm not sure if that's what you're referring to as the "base game", but the guns provided there do work as you describe. There's no rules for intermediate ammunition forms between the two you describe. Realistically, I would just use the rules for advanced firearm rifles for minie balls. Those represent weapons that are faster to load and dangerous at a longer range than the weapons in the Early Firearms category. There's not really a lot of mechanical room to play with between the two categories. You could perhaps give the weapon a range increment of 60 feet, as a midpoint between the two, or have two-handed versions take a Standard Action to load as a default. Realistically, if you're looking to model firearms with that much nuance, you may be looking to redesign the way firearms work anyway. (Note that Gunslingers as written are designed to work with Early Firearms as the tech level; if you put more powerful guns in the game, you make the Gunslinger class better.)
"Rasputin Must Die!" from the Reign of Winter AP provides rules for a bunch of early 20th-century weapons. Those can be found here.
The Technology Guide has rules for sci-fi guns (like Laser Pistols and the like), but that sounds like it's not what you're looking for in terms of "expanding on firearms".
A lot of 3rd-Party stuff predates Ultimate Combat, and tends to present firearms that work totally differently than Paizo's (somewhat idiosyncratic) version. None I know of provide a more detailed breakdown of weapons than Paizo does. Most post-UC 34d-party content focuses on expanding gunslinger options or making guns work for other classes over filling out their weapons table.