Aazayzel is a Demon lord of knowledge and the greed for knowledge. When a warlock makes a pact with Aazayzel he offers them the opportunity to buy his revolver. The price for such a powerful weapon is not cheap however, and he will either require the player to sell a portion of their soul or every good memory the player has ever had. The revolver at first glance is simple and seems to have no special ability. In the hands of anyone who does not have a pact with Aazayzel the gun is a simple 2d6 piercing, 3lb, ammunition, range of 50/150, 6 shots (reload). The gun behaves as a normal revolver and a character can become proficient with it it as normal. 

When wielded by a warlock of Aazayzel the gun becomes attuned and the wielder is able to channel their life force into the gun to cast ranged magic attacks. The attack cannot be a concentration spell or have any area of effect. The attack must be directed towards a single target within range and line of sight. The wielder can cast higher level spells through the gun for an increased amount of damage but the gun will require higher amount of life force channeled into it. 

When a spell is cast through the gun the wielder will take 1d6 + (spell level) to gain an additional 1d10 (piercing damage) toward the spells total damage per attack. The user must make a ranged spell attack as normal. On a miss the spell is lost and the user takes damage for nothing lol. 

Example: A player cast Eldritch Blast channeling through the gun (Eldritch shot) the player first rolls 1d6 and gets a 4. The player take a total of 4 damage because Eldritch Blast is a cantrip. The player then makes a normal ranged spell attack against an orc and rolls above its AC resulting in a hit. The player finally rolls 1d10 force as indicated by Eldritch Blast as well as an additional 1d10 piercing of the gun. The player is very lucky and rolls two max rolls for a total of 20 damage killing the level 1 orc in one hit

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: Bonus: Ranged Spell Attacks, Damage: Piercing, Aazayzel's Revolver, Requires Attunement - Pact with the Demon lord Aazayzel - Attunement description by a Warlock , Range, Ammunition (Firearms), Reload, Sap