There is another option here.
The metal cartridge description is as follows:
Metal Cartridge: These sturdier versions of alchemical
cartridges serve as the ammunition for advanced firearms.
They can hold either bullets or pellets.
This description from UC doesn't explicitly say anything about affecting the reload speed of a firearm. It only states that these cartridges are "sturdier versions of alchemical cartridges." I believe the comparison to alchemical cartridges is drawn here simply because the metal cartridges are more similar in structure to alchemical cartridges(paper packages that contain the bullet/pellets, wadding, and gunpowder in one easily handled unit), not because metal cartridges are meant to retain the faster reload capabilities of alchemical cartridges.
I would therefore propose that advanced firearms simply use a move action to reload to full-capacity, as stated in UC. Furthermore, with regards to the Rapid Reload feat, I would say we should apply it to advanced firearms (move action reload) in the same way that it is applied to the hand crossbow and the light crossbow (also move action reloads), bringing the reload action for advanced firearms down to a free action.
Now at first this might seem like it makes the 6 round capacity of the revolver useless, making the rifle a clearly superior option so long as you have the Rapid Reload(rifle) feat, but I would argue against that. First of all, the 6 round capacity is especially useful for firearm users with Two-Weapon Fighting feats, it allows them to make a full-attack with a revolver in each hand without needing another free hand to reload their weapons. Second, I would argue that using a revolver saves you from having to take the Rapid Reload(revolver) feat in order to make full-attacks, allowing you to take something else like Deadly Aim, Rapid Shot, Point-Blank Shot, Far Shot, Two-Weapon Fighting, or another feat that complements your revolver(s). Third, if two-weapon fighting isn't your thing, you could use your spare hand for something else, like wielding a shield, climbing, holding a torch, etc., all the while retaining your ability to make a full-attack with your revolver before needing your other hand free to reload. You could even make a full-attack with your revolver while being grappled because it only takes one hand to use.