@Ancalagon I think that's a terrific approach. The whole 1/encounter ability, while yeah it's abstracted/meta-gamey, works really well and is clear for players to understand.
Personally, I wrote up my own rules for smokepowder weapons. They're high damage, moderate range weapons that have some limitations.
1st limitation = This is Faerûn / Sea of Fallen Stars focused, where the firearms are mostly exclusive to Pirate Isles who traded with the gnomes of Lantan before the portal closed. So while there are guns circulating, and bullets a plenty, smokepowder is as rare as uncommon magic items, and costs 100 gp per packet worth 5-shots.
2nd limitation = Proficiency with them is not covered by martial weapons, so you need to get it through a special rules variant, subclass feature, Gunner or Weapon Master feat.
3rd limitation = Misfire and jam on natural 1. Also exposure to water can "jam" the weapon, and there is some sailing / underwater adventure in my game-in-the-works.
Anyhow, functionally it focuses on exploding damage dice, and pistols / muskets having two damage dice to increase odds of exploding.
Pistol (250 gp): 2d4 piercing damage; range 30/90; Ammunition, Loading, Exploding (reroll any damage die at max value and add to damage), Misfire (on natural 1 to attack, the pistol misfires and jams, cannot be used again until tended and cleaned during a short rest).
Musket (500 gp): 2d6 piercing damage; range 40/120; Ammunition, Loading, Exploding (reroll any damage die at max value and add to damage), Misfire (on natural 1 to attack, the pistol misfires and jams, cannot be used again until tended and cleaned during a short rest).
EDIT: I ran a quick test of 100 damage rolls for pistol and musket – not counting critical hits or misfires – compared to the hand crossbow (1d6) and heavy crossbow (1d10), respectively. From that little test, the pistol appears to do +2.75 damage more than the hand crossbow, while the musket appears to do +2.4 damage more than the heavy crossbow.