Firearms requires their own proficiency, gained only by being an Artificer or taking the following feat:

• You gain proficiency with Firearms.
• If you roll a misfire on an attack with a firearm, you can use your reaction to roll a d20. If the number rolled is higher than the weapon’s misfire score, the firearm does not misfire (but still misses). You cannot use this feature of this feat again until you complete a short or long rest.
• When you use the Attack action and attack with a one-handed weapon, you can use a bonus action to attack with a loaded firearm with the light property you are holding.

Following are the firearms used in Ozel, as well as their stats:

*Reload: The amount of shots you can fire before you have to spend an action reloading
  Misfire: When you roll your 1d20 to attack and get the Misfire number or lower, 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. Creatures who use a firearm without being proficient increase the weapon's misfire score by 1.

A gun cannot be magical nor deal damage that bypasses a creature's resistance to bludgeoning, piercing, or slashing damage from a nonmagical weapon. The exceptions are weapons created by Artificers.

Artificer level 12:

The firearm can deal damage that bypasses a creature's resistance to nonmagical weapons. The misfire score, however, is increased by 1.