Atonement

You have a +3 bonus to attack and damage rolls made with this magic weapon. When you hit a fiend or an undead with it, that creature takes an extra 2d10 radiant damage. 

Whenever you hold the rifle in both hands, it creates an aura spanning 10 feet around the user. You and all creatures friendly to you in the aura have advantage on saving throws against spells and other magical effects.

This weapon regains 1d6 charges every day at dawn. if the weapon ever reaches 0 charges you must roll 1d20, if you roll a 1 or lower the gun breaks. The first time this happens it can be fixed by an artificer of 10th level or higher. However, if it breaks again, it is disintegrated leaving nothing but ash behind.

Power Shot (1 Charge). You imbue 3 bullets with the holy energy of this weapon causing the to merge it a stronger bullet. This causes the attack to instead deal 4d10 piercing damage + the normal damage mod. If you land a critical hit the Damage dice only increases to 6d10.

Banish (1 Charge). You may infuse 1 bullet with the Banishment spell regardless of whether you can cast it. This bullet, upon impact, will cast Banishment while still dealing it's standard damage.

Bless (1 Charge). You may instill the Bless spell into a bullet allowing you to cast bless from a further distance. If you attack a creature with this bullet it must be one of the targets of the spell. The spell only triggers upon impact. If you shoot a fiend or undead with this ability the attack will do an extra 1d6 radiant damage

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, Anyone with a good alignment and a firearm proficiency, Damage, Combat, Range, Two-Handed, Ammunition (Firearms), Reload, Slow