Had some back and fourth about Air Cartridge Firing System and whether or not it replaces your normal loading mechanism with magazines that use pellets. It doesn't explicitly state it does, but I also don't see any examples of air pellets being sold in any way other than in magazines but then again I also don't see any examples of magazines other than those specifically labeled as "Magazine (Air Repeater)".
Any thoughts on this, or has Paizo written any clarification?
Firearms equipped with Air Cartridge Firing Systems don't fire pellets. They fire the same bullets the firearm usually fires. The only difference is that the bullets are propelled now by compressed air rather than black powder. This allows the firearm to be fired underwater, at the cost of a bit of range and the damage bonus from Kickback if your weapon has it.
It's basically a lower-level version of the Underwater Firing Mechanism, not an upgrade to turn any firearm into an Air Repeater.
Yup. The firing component is integrated into the cost of the ammunition in this edition. Normally for a black powder firearm that's gonna be black powder, but in the case of an Air Cartridge Firing gun the air cartridge replacements take that 'cut' of the cost instead. Since ammo for guns is generally sold in bundles of 5 or 10 anyways, just handwave it as that's how many shots one canister is good for, and replacing it is part of the reload action. Or you could handwave it as a gun with the firing mechanism takes smaller compressed air ampoules that are good for one shot, typically bundled with the individual bullets a la paper cartridges.