The Spell Cannon - A large two handed, lever-action blunderbuss, with intricate silver linings along the edge of the muzzle. Arcane symbols and lines, run up and down the sides of the weapon, originating from a large bronze mechanical piece in the middle of the blunderbuss, containing a hole for its designed shells to be slotted. Pulling the lever down ejects previously used ammunition form the chamber, while pulling the lever forward pulls a new shell into the chamber.

The Spell Cannon can hold up to 2 shells in its chamber at once, with the previous shell needing to be ejected via a bonus action before another can be primed for shooting. It takes an action to load to new shells into the chamber. 

Spell Shells - When a spell is aimed at a creature wielding this weapon, they can use their reaction to absorb the spell into an empty shell on their person, or loaded in The Spell Cannon's chamber. If the spell aimed at the weapon wielder is of 4th level or higher, the shell will explode, jamming the weapon if it is in the chamber. If it is casting a spell of 3th level or lower, make an ability check using your DEX. The DC equals 10 + double the spell’s level. On a success, the creature’s spell is fully absorbed into the shell as it begins to glow and becomes an Active Spell Shell. On a failed save, the shell explodes, and the targeted creature gains disadvantage on any associated ability checks for the spell.

Any Active Spell Shell when fired from The Spell Cannon, will activate the absorbed spell immediately upon impact with an object or creature. Any targeted creature will have disadvantage on any ability checks triggered by an active Spell Shell and the weapon wielder has advantage on any attack rolls made by the spell.

Inactive Spell Shells can be fired from the gun, only dealing its normal damage without any added effects.

1 Spell Shell can be crafted over 3 hours, using materials that are relative in value to 75 gold.

 

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: Magic, Damage, Control, Debuff, Utility, Combat, Consumable, Eldritch Machine, Artificer, evocation, divination, transmutation, enchantment, Range, Two-Handed, Ammunition (Firearms), Reload, Push