Arcane Hand Cannon

This sleek adamantine weapon has a grooved soft leather grip, and a cylinder made of thick and layered glass. This heavy revolver stores spells cast into it, holding them until the attuned user chooses to use them. The hand-cannon can store up to 5 levels worth of spells at a time.

Any creature can cast a spell of 1st through 5th level into the hand-cannon by touching it as the spell is cast. The spell has no effect, other than to be stored in the weapon. If the hand-cannon can't hold the spell, the spell is expended without effect. The level of the slot used to cast the spell determines how much space it uses.

While holding this weapon, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell. The spell cast from the hand-cannon is no longer stored in it, freeing up space.

The hand-cannon can also be used as a ranged weapon at anytime. It is a ranged martial weapon with a range of 40/20, and it deals 2d8 piercing damage (magical). Dexterity or Intelligence is the ability modifier used to make a ranged weapon attack. If the last spell that was cast from the hand-cannon was a damage dealing spell, the weapon attack changes to the same damage type of that spell. This effect lasts until a new spell is cast from the hand-cannon, or until the attuned user finishes a long rest. 

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: martial weapon, very rare (requires attunement), Damage, Combat, Range, Ammunition (Firearms), Reload, Sap