Black Powder Mastery

This takes the renaissance items from the Firearms section of the Dungeon Master's Guide and extends on it:

The weapons of the renaissance Items get a new property: Misfire 3

Misfire. Whenever you make an attack roll with a firearm, and the dice roll is equal to or lower than the weapon’s Misfire score, the weapon misfires. The attack misses, and the weapon cannot be used again until you spend an action to try and repair it. To repair your firearm, you must make a successful Tinker’s Tools check (DC equal to 8 + misfire score). If your check fails, the weapon is broken and must be mended out of combat at a quarter of the cost of the firearm. If the attack roll is a 1 then, in addition to the weapon jamming, increases its misfire score by 1d4.

You can attempt to maintain the weapon to reduce the weapons chance to misfire. If u spent at least a hour using your Tinker's Tool and a successful Tinker's Tool check (DC equal to 8 + base misfire score) you can reduce the misfire score by 1d4 to a minimum of its base misfire score.

If you want to increase the reliability of the weapon even further you can take 3 working day, halve of the cost of the firearm and a successful Tinker's Tool check (DC equal to 8 + misfire score) to reduce the base misfire score by 1. Proficiency with Tinker's Tools, Smith Tools, Woodcarver's Tool or the knowledge of the Mending spell decreases the DC by 1 each.

Creatures who use a firearm without being proficient increase the weapon’s misfire score by 3.

Crafting a Firearm is a new profession and the techniques are not fully developed or the gunsmith have to improvise with the materials he got at hand. If you craft a firearm yourself you determine its base misfire score by rolling a Tinker's Tool check and apply the base misfire score relative to the roll:
0-5     => Misfire 6
6-10   => Misfire 5
11-15 => Misfire 4
16-20 => Misfire 3
21-25 => Misfire 2
26+    => Misfire 1

Add "Crude Bullets (10)" to the list of renaissance items. They have a cost of 5 silver and have the property: Crude Ammunition

Add "Precise Bullets (10)" to the list of renaissance items. They have a cost of 10 gold and have the property: Precise Ammunition

Crude Ammunition. Whenever you make an attack roll with a firearm using this ammunition, increase the weapon's misfire score by 2 and reduce the weapon's range by 10 feet.

Precise Ammunition. Whenever you make an attack roll with a firearm using this ammunition, increase the weapon's range by 15 feet.