Well, first of all, the question about why develop firearms rules was made ironically. In any case, I assume that in a fantasy world everything is possible. For example, why not to develop a sub-machine gun type device which fires rounds of Magic Missiles, shotguns with Burning Hands effects or a Fireball grenade launcher? We still be anchored in a Lord of the Rings type fantasy. Anyways, here firearms are invented (Alkenstar), and if I remember well, were invented due to the lack of magic. So if they want to compete with chain lightnings and scorching rays, they must upgrade them. In any case, i just want to have fun with firearms and even the new class Gunslinger (not available yet) does not fulfill my desires.
I had a character concept for low-to-mid level campaing in which I would play an Alkenstar Commando (fighter / rogue) with a rifle and a support close combat weapon. Under the 3.5 rules, rifle (percussion firearm) can be reloaded with a move action per shot. With the rapid reload feat, I could make full-attack shots and improve its damage with feats like deadly aim and also defend myself in close combat. With that new rules I must be Gunslinger to optimize firearms and I would depend on my Wisdom score to make those deeds (like reload as a free action, which still restricts my ranged attacks a lot). So, a nice character concept but absolutely less powerful than the other characters in the group, making it less fun to play.
I was trying to build an effective and different character using t'he background history of Alkenstar, but seeing how firearms and technology work, I will re-think the concept and return to a standard character with a standard background. We are not talking about having a character with an M4 Assault Carbine or an M241 Minigun, just a weapon with a fire rate similar to a crossbow with firearm flavour (i.e. the exploding dice and misfite rate). Unfortunately, my GMs aren't fans of housemade rules (principally to keep the game balanced), so in a fantasy world not everything is possible.
Answering some comments, feat Crossbow Mastery requires a crossbow proficency, is applied on a crossbow type and I haven't seen any exception mentioned to use it with firearms; about Arcane Archer, all class features are related to arrows (not even bolts for an Arcane crossbowman); 150 ft is the first range increment for the rifle (of a minimum of 4 increments).
I insist in that's a personal view of a player who is not allowed to try housemade rules made in cooperation with the GMs. I'm sure most players have arranged this with extra rules, forbidden them or use them what they are in this kind of fantasy sets: a limited support ranged weapon.