So a cleric uses BWP while holding, say, a musket. Does he now attack touch AC (within the proper range, yada, yada) as does a Gunslinger, albeit sans Grit? Or does he just fire the thing as if it's a crossbow, targeting regular AC w/out penalties?
He has proficiency with the weapon, he follows all rules for using it.
Guns are not the exclusive purview of gunslingers, like bombs are to alchemists.
Anyone who takes 'weapon proficiency (pistol)' can pick up and fire a pistol, and they use touch AC within the first range increment.
This is similar to picking up a bastard sword and using BWP, you get the proficiency with the weapon, which means you can wield it one handed. Or picking up an orc double axe and being able to use it as a a double weapon.