I suggest the Musketeer swashbuckler archetype, not only does this give you Rapid Reload with muskets. However, it also provides you with Quick Clear, which is important.
Also provides you with Weapon Finesse with Rapiers, so you can have Dex to Damage at level 1 as a Human if you want. Of course the downside to using a musket is that you can never fire them as a full round action.
Still, if you are planning on just using the gun as your initial volley then I suggest Deadly Aim and Opening Volley as feats. You might not need Precise shot if you are just going to shoot at them, but if you do not mind going DIPTASTIC you could take 2 levels of Divine Hunter Paladin. Level 1 gives you Precise Shot for free, level 2 gives you Cha to saves which is much better than going for Charmed Life.
Quick Draw could be useful too,
Once you have those levels, you could even go pure Fighter for the feats or go back to one of your initial classes.
The Opening Volley feat pretty much matches what you want to do to a T.
I definitely recommend quick draw.
I forgot that Dagger pistols could be thrown. With the blinkback belt you could actually TWF with thrown Dagger Pistols.
Quickdraw a dagger pistol, reload it as a free action, Fire it, reload it, fire it again (rapid shot), and then throw it. This works because the Dagger Pistol is considered a double weapon. End of the turn it warps back to your sheath.
Full attack rotation at level 20 with TWF, a blink back belt, and haste is
Based on the rule that (If you get multiple attacks because your base attack bonus is high enough, you must make the attacks in order from highest bonus to lowest. If you are using two weapons, you can strike with either weapon first. If you are using a double weapon, you can strike with either part of the weapon first.)
Draw dagger pistol one, Gunshot 1 (Main Hand Full BAB), reload pistol 1, gunshot 1 (rapid shot), reload dagger pistol, gunshot 1 (haste), reload dagger pistol one, draw dagger pistol two, gunshot 2 (Off Hand Full BAB), Gunshot 1 (Main Hand BAB -5), Throw dagger 1 (BAB -5), reload dagger pistol 2, fire dagger pistol 2 (BAB -10), reload dagger pistol 2, throw dagger pistol 2 (BAB -10), Draw dagger pistol 3, gunshot 3 (BAB -15), reload dagger pistol 3.
Which is a pretty neat attack layout even if it costs you 300k for 6 +5 weapons >_>
I swore there was a recent feat that removed that penalty, even if there wasn't there is still the option of going for Possessed Hand into Hand's Autonomy
Rapid Reload
Point Blank Shot
Rapid Fire
Quick Draw
Two Weapon Fighting Chain
Possessed Hand
Hand's Autonomy
If you were a human trench Fighter 1, Mysterious Stranger 1 into Musketeer 1 into a trench fighter you could have all the feats you need by 7th level.