This is a Titan Hunter automatic rifle on a hit does 6d4 piercing damage. Burst fire is negated for this weapon.

Can be used to bring down the huge behemoths known as titans. Imbued with elemental energy, this weapon can fire rounds of any element.

Ammo: Elemental round: generates from elements that are near by in a 10ft radius, round type can be switched during battle and during short and long rests. Magazine can hold 20 rounds. Ammo reloads automatically when magazine is empty and is generated from the element in use. 

Misfire: when the attack roll is lower than the weapons misfire score, the elemental round being used can’t be used until you use an action and roll above its misfire score . If only one elemental type is present and no other type is, then the weapon can’t be used until you use two actions and roll above its misfire score twice. 

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: Ranged Weapon Attack: Piercing, Invisible, fighter or ranger and have firearm proficiency , Damage, Combat, Range, Two-Handed, Ammunition (Firearms), Burst Fire, Reload, Slow