Well, we already have a lot of late medieval items, and we even have Samurais and Ninjas, so it's too late to worry about the look of the game.
I could live with early medieval guns that need some kind of fork to be planted into the ground, which needs time to be set up, fixes you on one spot, and limits your aiming to a cone of perhaps 10° in all directions. This means you have to plan your shots very well, and you are really vulnerable while shooting. The damage should be a bit better than for arbalests.
Compared to the arbalest it should be of about the same effectivity. Arbalest is faster reloaded (yes!), and you can move while aiming, musket has little to no bullet drop, higher missile speed and damage, but you can't move while shooting, you need time to shoot and you can't aim properly.
I think I could live with that.