The Fusion Cannon is Megatron's ultimate weapon. It is attached to his right arm and is devastatingly powerful, as demonstrated by its sudden removal of Ironhide's head. (It's at least as powerful as Fortress Maximus's hands, then.) It strikes fear into the sparks of both the Autobots and the Decepticons, for any who challenge it will surely fail. Except Optimus Prime. Only he appears to be able to survive a shot from the fusion cannon, considering how many times Megatron has shot Prime. Oh, and Starscream.