Weapon (武器, Buki) is an extra chapter included with Volume 11.
One year ago, Metal Bat finishes off another monster, but breaks another one of his bats. He is later told that he is being promoted to S-Class and that he can be given whatever he wants. Metal Bat asks for a metal bat that will not break no matter what. When he gets home, he is scolded by his sister for not getting Sweet Mask's signature.