This weapon is designed and created with the possibility of customizing the weapon for each situation. Different modules, such as specific stocks, sights, barrels, grips and actions can be swapped out and changed as desired - doing so requires a short rest and access to Tinker's Tools.

Module Slots and examples:
Barrel
Long Barrel (Range 100/400, Two-Handed),
Short Barrel (Range 40/120)

Receiver 
6 Shot Cylinder (2d8 Piercing),
6 Shot Elemental Action (2d8 Elemental - Elemental damage type can be changed once per round),
6 Shot Specific Action (3d6 Elemental - Elemental Damage type specific for cylinder),
6 Shot Explosive Action (2d6 Fire to target and creatures within 5ft of target)

Sight
Iron Sights (No Disadvantage against melee attackers),
Eagle Eye Scope (Increases both range increments by 75ft),
Night Scope (Gives Darkvision through scope to maximum range)

Stock/ Grip
Pistol Grip (When used with Short Barrel, weapon is 1 Handed),
Rifle Stock,
Pistol Grip with Stock Butt
Hair Trigger (On a critical hit, the weapon fires a second time if there is still available ammunition in the cylinder)

Barrel attachment 
Bayonet (1d8 Piercing, 5ft),
Barrel Light (Emits bright light in a 30ft Cone, and dim light for an additional 30 ft, able to close to stop light from shining through),
Bipod (Gives advantage in all ranged attacks fired from prone within the first range increment),
Firecone ( Each creature in a 15-foot cone must make a Dexterity saving throw. A creature takes 3d6 fire damage on a failed save, or half as much damage on a successful one. 1 Use/ Short rest)
Suppressor (Makes weapon audible only out to 30ft, stops visible flash from barrel)

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: Requires proficiency with firearms, Range, Ammunition (Firearms), Reload, Sap