Throw in deadly aim and rapid shot (maybe clustered shots if you dislike managing ammo types) and you have all the useful feats for a gunslinger.
Mysterious stranger never gets to add Dex to damage like most gunslingers do, but it does get to add Cha to damage - as a swift action, and costing 1 grit for 1 round. It's a great one-level dip, terrible main class.
There's a magic item which can reload as a swift action (alchemical cartridge + rapid reload on a standard musket = move action) but you're probably looking at a dragoon musket or an air repeater if you're not playing a musket master. Those can hold more than one shot (3 or 6 respectively) before needing to reload.
If you want to shoot people with a standard musket then musket master is it. Reloading as a free action rather than move action is just that good.
If you want a social gunslinger look at the maverick archetype instead.
If you want a gun user which adds Cha to its grit pool then dip in one of the classes with a compatible pool (luck or panache) - swashbuckler, hooded champion ranger, sleuth investigator, kata master monk.