A Broken cannon barrel is used when repairing Dwarf multicannons within the Artisans Workshop. It can be picked up from the cannon barrel desk in the repair room after the base and stand have been repaired. It can be repaired on an anvil with a hammer in three stages, eventually giving a fixed cannon barrel.
This item cannot be taken outside of the repair area. It can be re-obtained from Isak if you have left the area after obtaining it.