Armcannon

An armcannon is a special weapon designed for the use of Warforged, converting their internal battery supply into an energy attack. This barrel-shaped device must be custom-fitted to a Warforged in order for them to attune to it, a process which takes 1 hour (which must be done before the Warforged can attempt to attune to it). If it is fitted properly, it slides over the Warforged's arm and locks in place just above the wrist. With a twist, however, the outer casing comes loose and is able to be slid forward, covering the hand.

While attuned to this item, a Warforged can use a Bonus Action to activate the device, rendering the hand it covers unusable until the weapon is deactivated with another Bonus Action. While the device is active, it has the same statistics as a Laser Pistol, except the wielder is considered proficient with it. However, each time an attack is made with the weapon, the wielder must expend one hit die unless an energy cell is loaded into the weapon. The weapon can be fired 15 times before an energy cell must be replaced, after which any further attacks expend hit dice as normal.

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: Warforged, Wristwear, Range, Ammunition (Firearms), Reload, Vex