Arcane Rifle

While it may look like an ordinary hunting rifle, the Arcane Rifle works via a completely different principle. Once created, an Artificer can inscribe runes on the barrel that provide magical energy for creating and shooting small, bullet-shaped charges of various elements.

 

The basis of its construction is a common Hunting Rifle. With the use of Smith's Tools, a magical sigil can be etched into the rifle's barrel. In short, it is a modern equivalent to an Arcane Firearm. Thus, it will not function as one without a specific Arcane Infusion (Repeating Shot). Other than that, the two function in pretty much the same way, save for some minor differences, such as the need to physically pull the trigger and the know-how of firearm use.

Spell Mode: While in this mode, the rifle functions as a spellcasting focus for ranged damage spells. In short, it acts as an arcane firearm, adding a d8 to spell damage rolls.

Fire Mode: When not in Spell Mode, the rifle fires the magical rounds provided by the Repeating Shot infusion, adding +1 to attack and damage rolls.

Note, that existing Hunting Rifles can also be converted into Arcane Rifles. Such conversion constitutes a long rest or a downtime action lasting at least 6 hours. First, you need to etch the sigil into the rifle's barrel, then activate it (DC 16 Arcana Check) for it to work properly. 

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, You can use an Arcane Rifle without attuning to its magic, of course. Just, it won't do much. , Creation, Combat, Artificer, Range, Two-Handed, Ammunition (Firearms), Reload, Slow