The Crossbow Barrel is an unobtainable barrel attachment in Unturned. It reduces gun noise by 60%, and it is only found in the Crossbow internally.
The Crossbow barrel was never meant to be an item that could be in the Player's inventory. In the inventory, it will appear as a few grey slots. It can be removed without dying by looking for it on the inventory and dropping it the same way a regular item would be dropped. Its description file is completely empty, and it has no model for it.
This means that it is more of a modifier than an item.