Gunpowder or smokepowder can have its place especially in a renicance-like era setting like Forgotten Realms, as long as they are treated appropriately. To do that I think they have to be relegated to fire and forget or first strike weapons.
Here is how I treat them in my game:
The pros:
they are touch attack weapons,
pistols do 2d6, rifles do 2d8;
The cons:
It takes a half minute to reload
they are expensive 200g+ for a pistol,
they are delicate, on a roll of natural one the gun breaks and can not be used again until repairs are made
They require a seprete craft skill (gunsmithing) to build and repair.
Only humans and gnomes currently build them.
Ammo is heavy, it requires a one ounze bullet and one ounze of smoke powder to fire a single shot. Meaning just to carry 16 shots is going to weigh 2lbs
I think there is some other stuff but its late and my frontal lobe is starting to shut down for the evening.