The Bazooka is an upgrade for Miles "Tails" Prower in Sonic Adventure 2 and Sonic Adventure 2: Battle. It is an a weapon attachment to Tails' Cyclone that allows its Volkan Cannon to break iron craters. It is the equivalent to Dr. Eggman's Large Cannon.
The Bazooka is found in Eternal Engine and is required to beat the level. After passing the fourth Star Post, head until Tails arrive in an outside segment in space with a long bridge and a switch at the other end. Under the switch is a locked door and a floating platform between you and thevdoor. The Bazooka can be found on the other side of the locked door.
To open this door, you can either walk across the bridge to activate the switch on the other side which will open the door. If you to fail to reach this switch by falling off the bridge, let Tails fall down on the large platform found below and a switch should be found in the back, which should also unlock the door. You can then get back the former platform by hopping onto the platform nearby to ascend back up to the door. Either way, the door will be unlocked and the Bazooka can be found in a circle of rings.