An Armsheath functions as a Prosthetic Limb, but also grants the following additional benefits:

• You can insert a one-handed ranged weapon no bigger than the arm into the sheath as an action. The sheath can hold up to three at a time.
• You can retract or extend a weapon from the sheath as a bonus action. While a weapon is extended, you can use it as if you were holding it, but your hand remains free.

It is up to DM discretion what weapons are compatible with the sheath. For example, a Hand Crossbow would be obviously two large to fit in the arm, but a pistol or other firearm may work.