Aether Carronade

An Aether Carronade is a magic weapon made through artifice that deals force damage. It attaches to your arm, becoming inseparable from you as long as you're attuned to it. To attune to this item, you must hold it against your forearm for the entire attunement period.

As an object interaction, you can retract the Aether Carronade into your forearm or extend it from there. While it is extended, you can use the weapon as if you were holding it, and you can't use that hand for other purposes. 

This arm-integrated ranged weapon can use your life force to amplify attacks made with it. You may sacrifice hit points and add +2 to the total damage roll for every 1 hit point sacrificed (maximum of 5).

If you are a warforged, artificer, or autognome, you may use the Aether Carronade as your spell focus. This weapon ignores the reload and ammunition weapon properties.

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: Magic, Replace Damage Type: Force, Ignore Weapon Property: Loading, Ranged Weapon Attack: Force, Proficiency: Firearms, warforged, artificer, autognome, or into a prosthetic limb , Range, Ammunition (Firearms), Reload