Rare makes a poor balancer, especially in an open ended game where people *refuse* to play unless they can get all their desired features on one map. (Why can't I find a starting location with HFS, Iron, Flux, Magma, Running water?). Add saltpeat and sulfur to the mix and it won't be 'rare' so much as you'll see people complaining about how long it takes to generate a map with all of it at one place.
As for the people complaining about 'why use something that's dangerous', take a look back at the historical reasons. Basically, it's easy to train up a competant gun user. Sure your xbows do more damage(?), have better range and accuracy, don't blow up, never misfire, and can be build from a treecap and a dead rabbit, but it takes a year to train them up. The advantage to going through all the crap to get gunpowder is that you can draft your haulers as emergency militia and give them even odds of doing something useful. Sure the guns'll blow up in a few hands, but they're just haulers. peasants...
Plus, as mentioned, the smoke effects would be cool
dreiche its pointless to add guns imho. Firstly, I dont consider guns "fantasy weaponry". What request will be next? Cannons?
Secondly, ranged weapons are already overpowered. Also your suggestion is making no sense, why should melee units use a ranged weapon for first attack..and if they do use it, why should that be a gun instead -for example- a hand x-bow?
Cannons are actually a MORE appropriate weapon for the setting. The oldest surviving cannon dates to 1288, well within our setting period.
Xbows require the long spars to create the force. Thus, a handbow would be extremely weak. In a firearm, thus size of the weapon only increases accuracy. It is a staple of swashbuckling fiction that the fighter fires off rounds from a few pistols before engaging in melee. (Read 3 Muskateers)
If you want to go back even further, it was a standard barbarian technique to use large throwing axes as an initial strike before engaging in melee. These were often used to lodge into the wooden shields, weighing them down.