The Large Cannon is an upgrade for Dr. Eggman in Sonic Adventure 2 and in Sonic Adventure 2: Battle. It is a weapon attachment to the doctor's Egg Walker that allows its Volkan Cannon to break iron craters. It is the equivalent to Miles "Tails" Prower's Bazooka.
The Large Cannon is found at the beginning of Weapons Bed and is necessary to complete the stage. When beginning on the carrier, head down the path until you get to the end of it. There you will find the control center on the left, with three small room in the building, each blocked with dynamite. Head into the roon in the middle and the Large Cannon can be found just behind the door.