A few revisions and expansions to firearms in this setting.
First, the damage. Namely, it is just too high. Practically every weapon in D&D does damage in the 1-12 range, from 1D4 to 2D6. My firearms just blow this out of the water. There is really no good reason NOT to have at least one gun on anyone trained in their use. Being able to pre-load a rifle and get off one shot that has the potential to kill 2 or 3 HD worth of monster in 1 hit is just too good to pass up. No matter the concerns around effectively using firearms, everyone would probably end up carrying one.
So let's put firearms back into perspective with the rest of weapon damage.
.55 pistols: 2d4. Appropriate damage for a one handed weapon, on par with a longsword but with better average damage. This damage is appropriate for a pistol (1 shot of 2d4) and sword (1d8) fighting style, rather than just toting around a piss load of pistols (old damage at 2d6+1).
Caliver and Rifle: 2d6. Appropriate for a 2 handed weapon. Anyone who takes the time to fight with a caliver or rifle and work through reloading and cleaning it should be able to dish out good damage on par with any other well armed combatant. Also only somewhat stronger than other ranged weapons that rely on physical/mechanical strength.
Musket: 2d6+2. For the gunner who wants to handle these lumbering weapons, they can do excellent ranged damage that would rival a high strength fighter swinging a two handed sword.
How do weapons interact with other D&D mechanics?
For spells, a cantrip would be effective at disabling a firearm. A water cantrip would ruin powder and put out matches. Other cantrips could easily disable firing mechanisms as well. Warp wood and heat metal would destroy any firearm that was affected. A gust of wind spell or high winds from control weather could blow powder out of a matchlock without save and do the same to a snaphaunce on a failed save. Fog generating spells would affect firearms as if it were humid (+1 misfire chance).
On the positive side, a cantrip could clean a weapon and return the misfire chance to 1-in-20 instantly upon casting.
Fire effects that engulf a firearm would detonate a shot on a matchlock without save and on a snaphaunce on a failed save. Fire effects that hit a person but not their weapon would detonate a shot on a matchlock on a failed save but a snaphaunce would be safe. For example, a fireball would engulf a weapon but a burning hands spell would only hit the gunner.
An electric shock, such as being affected by a shocking grasp or lighting bolt, would detonate either firearm without a save.
Powder that is contained in a hard leather pouch should be safe from most affects. Even if a fireball hit a gunner, the powder shouldn't ignite since there wouldn't be sufficient air in a pouch to ignite its contents. A soft cloth pouch, however, could very well flash away and turn the powder inside into a nice explosive. A constant flame could do the same to even a hard leather pouch given enough time.
This just gives a general idea on how to adjudicate how firearms relate to the game world.