In replace for an arm you get an arm attached rifle equivalent to your upper arms weight with no need for reload and fired at the will of you mind. The rifle has magic runes attached to engraved on its barrel and can shoot certain magic spells.
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.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Slow. If you hit a creature with this weapon and deal damage to it, you can reduce its Speed by 10 feet until the start of your next turn. If the creature is hit more than once by weapons that have this property, the Speed reduction doesn’t exceed 10 feet.
Notes: Disadvantage: Acrobatics, Range, Two-Handed, Ammunition (Firearms), Burst Fire, Reload, Slow