I think running around with a single-shot musket could get very boring very quickly and end up with me charging with my bayonet. Don't you think?
I second that.
The closest would be maybe dishonored. Even though we are a few centuries later, Corvo stil have a one shot weapon.And it felt really good. More like a shotgun than a pistol, but it was the kind of gun I'd use very rarely. And it was ok because they were a whole lot of option beside using our gun. But if I had to do the whole game with the gun only, I think it would quickly become tiring. The reload time would become frustrating because it would break the flow of the game.
But I just re-read your post, and you propose to have modern firearm (or at least decent) in term of efficacity, and a medieval looks?
This is not a bad idea, because if you go with the old arquebuse, I guess you would miss something like 90% if your shot if you are not at very close range, and that would'nt be very fun to play (my numbers are not accurate, but you get the idea )
In the other hand, if you introduce modern firearm, the whole world would have to adapt, and many medieval things would become obsolete (armor? close combat weapon longer than a knife? bow?)
But, the idea itself is interesting, it would be nice to have guns but not a steam-punk universe, for a change. It could be more like warhammer 40k. They have spaceship, gatling power sword, and other futuristic things but the feeling is more medieval than futuristic. So, for the good of the game and the rule of cool, I think you can dismiss some physics laws