The Cannon Arm parts was first found by Rickert and Erica in Godo's warehouse. It replaces Guts left forearm, which he lost during the Eclipse, the hand hides a small magnet in order to better handle the Dragonslayer, and above all it hides a cannon that many times saved the life of the Black Swordsman.
Guts operates the cannon by loading powder into the barrel and ramming the cannonball into the barrel with a special ramrod. Guts then pulls a rope/cord that pulls his mechanical hand below the barrel which then leads to the powder igniting and then firing. Guts makes use of this cord in situations where he cannot let go of the Dragonslayer or he can't use his right hand by pulling the cord with his teeth and firing the cannon that way.
While in berserker mode, Guts is able to move the fingers of the arm freely.
As it is a mechanical weapon, Guts must regularly maintain it with gunpowder and ammunition for both the cannon and crossbow to ensure its continuous use in battle as well as tools used to clean the barrel which he keeps in his bags.
Rickert came up with idea and created the Cannon arm for Guts as farewell gift before he left .
The Cannon Arm bears similarities to the Metal Gauntlet worn by Ashley J. Williams of "The Evil Dead" franchise and in the English translation of the Manga, Guts even used the infamous one liner "Groovy" to describe his new arm.