Arcane Revolver

Firearms are a new and volatile technology, and as such bring their own unique set of weapon properties. Some properties are followed by a number, and this number signifies an element of that property (outlined below). These properties replace the optional ones presented in the Dungeon Master’s Guide. Firearms are ranged weapons. 

To fire the arcane revolver, the player draws back the hammer, and then pulls the trigger. On pull, the hammer falls and contacts a small engraved rune. On contact, the rune releases explosive energy into the chamber that propels a projectile forward through the barrel. The revolver holds four silver ball-shaped rounds.

Reload. The weapon requires a full action to reload, and movement speed is halved for that turn. 

Gunslinger. The player can fire twice per turn.  The second shot does 1d8 piercing damage with no bonuses.

Fanned Shot. With full ammunition, the player can use a fanned shot to fire all four rounds at a single target. The player makes an attack roll for each shot. Each shot does 1d8 damage with no bonuses. 

Misfire. Whenever you roll a natural 1 on an attack with this weapon, roll a d20 with advantage. The player must succeed a DC10 dexterity saving throw or the weapon backfires doing 1d8 damage.  After a backfire, the weapon is unusable until the player fixes it during a short or long rest.  On a long rest, the player can fix it without making a roll. On a short rest, the player must succeed on a d20 roll with a DC of 5. On a natural 1, roll again. If the player rolls a second natural 1, the weapon is destroyed forever.

Ammunition. All firearms require ammunition to make an attack, and due to their rare nature, ammunition may be near impossible to find or purchase. However, bullets for the arcane pistol may be crafted with a set of Tinker's Tools using 24gp worth of silver (makes 20 rounds). If the player is proficient in Tinker's Tools, cost of crafting ammunition is halved.

It’s up to you to decide whether a character has proficiency with a firearm. Characters in most D&D worlds wouldn’t have such proficiency. During their downtime, characters can use the training rules in the Player’s Handbook to acquire proficiency, assuming that they have enough ammunition to keep the weapons working while mastering their use.

Notes: Ranged Weapon Attack: Piercing, Range, Ammunition (Firearms), Reload, Sap