This rifle appears to be made of solidified, constantly shifting, darkness, like a writhing hurricane stuck in a bottle. streaks of lightning may be seen occasionally, though they provide no light.

Anyone that attempts to touch this rifle that does not meet the attunement requirements is blasted back by a cloud of smoke that deals 6d6 thunder damage. This damage hits even if the one struck is resistant or immune to thunder. If the one struck is evil, they take double the damage. The rifle may be wrapped in cloth and move that way, though even if covered, an evil creature entering within 5ft of the gun is still attacked regardless of it being concealed. 

if a creature that meets the requirements attempts to touch the rifle, they will be wrapped in darkness and be forced to under go a hunt-or-be-hunted trial within a subspace the rifle creates. If the creature wins the hunt, they become permanently attuned to the rifle and may summon it to them with a thought.

This weapon magically generates its own ammunition, which appears on the butt of the rifle. 4 rounds are stored at once and regenerate at the end of each round. 

This weapon has a built in silencer that provides the wielder with advantage on stealth checks to remain hidden after making an attack.

This weapon deals 4D4 Lightning damage.

This weapon a range of 300/1200.

This weapon has a scope attached which provides advantage on long range shots. 

this weapon has the loading property.

This weapon's attacks are considered magical, and ignore resistances to Thunder and Lightning damage. If the target is evil, this weapon ignores immunity to Thunder and Lightning damage.

Dormant State

The wielder gains a +3 bonus to attack and damage rolls made with this weapon.
When the wielder hits with an attack using this weapon, the target takes an additional 4d4 thunder damage.
If the wielder hits with an attack using this weapon, they can choose one of two effects to trigger. Each effect can only be used once, and they cannot be used again until the wielder finishes a short rest:
A Fog Cloud spell centered on the target.
A Silence spell centered on the target.

Awakened State

The bonus to attack and damage increases to +6.
The thunder damage inflicted on a hit increases to 8d4.
An additional effect that can be triggered is added:
A Hold Person spell (spell save DC 15+PB).

Exalted State

The bonus to attack and damage increases to +9.
The thunder damage inflicted on a hit increases to 12d4.
The spell save DC for the Hold Person spell increases to DC 17+PB.
An additional effect that can be triggered is added:
The Blight spell (spell save DC 17+PB).

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: Dex 18, proficient in firearms, may not be evil., Range, Two-Handed, Ammunition (Firearms), Reload, Slow