Arcane Arm Cannon

At first, this magic item looks like a regular prosthetic arm, however, as a bonus action, you can speak the command word, transforming the lower arm into what resembles a metallic arm cannon with arcane runes etched into the metal. As an attack you can make a ranged attack, dealing 4d8 force damage upon hit. 

This weapon has 10 charges. After you hit, you can spend a charge to attack again. On a hit, you deal 2d8 force damage, and can expend another charge attack again, again dealing 2d8 force damage. You can use this feature as many times as you like until you miss with an attack. Once you miss, you cannot use a charge until the start of your next turn. All expended charges are reset on a long rest.

Notes: Ranged Weapon Attack: Force