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.
Presumably, this cannon is powered by some sort of nuclear fusion reaction. Sometimes Megatron has been able to link up extradimensionally to a black hole to augment this power, increasing it astronomically, and making him look real cool and space-rippy in the process. Most bodacious!
Galvatron had a big cannon, too, but it didn't work by fusion. That one worked similarly to experiments on dead frogs. No, seriously.
As part of an agreement with Cobra, Megatron's body was rebuilt to resemble a human tank, with his fusion cannon being replaced with a magnetic rail gun.
Generation One cartoon
Megatron could remove his cannon from his main body. One time, Brawn stole it from him and used it against him. Megatron was not amused.
Fusion cannons are also oddly useful when you just wanna kick your lieutenant for shooting you in the back but don't want to have to walk over to him...yeah, we don't question that.
The fusion cannon was a gift from Starscream. How ironic how Megatron decided to pay him back a few hundred thousand years later by blowing a huge hole in his torso .
- Megatron loved his fusion cannon so much, he had another fusion cannon installed on the Nemesis. The blast radius from the weapon is massive. Most kickass. It requires a power up time to fire again. Less kickass.
- Although not Megatron's, according to Beast Wars, the Ark's cannon was a fusion cannon.
Live-action film series
Megatron's cannon is presumably telescopic, as he keeps it stored retracted in his left arm when not in use. However, he did fire a blast from his right arm that sent Jazz flying in the air several feet, so it may be possible that each arm stores a fusion cannon and they combine for an increase in power. Its powerful blast is enough to knock Optimus Prime back three city blocks (although there was a building in the way) and requires Megatron to use both arms to steady the aim. Because Megatron is always concerned about collateral damage (or it could be so powerful that without balance it could knock him backwards a slight distance).
Revenge of the Fallen film
After returning from the dead, Megatron had a new fusion cannon that was stored in his right arm. The new cannon was shorter and larger than the old one and also had a bayonet-like spear attached to its underbelly. The most notable use of this cannon was during Megatron's forest battle against Optimus Prime, where he impaled the Autobot leader through the chest and then fatally blasted through him at point-blank range. Oh, and he also killed Sam Witwicky, but annoyingly for Megs, he was brought back to life. Later, while battling Jetpower Optimus Prime, Optimus uses Megatron's own fusion cannon to blast off half of his face before slicing off his right arm and blasting him through a wall.
It is also mentioned on Ironhide's packaging that he also has a fusion cannon; presumably it is the blue glowing laser weapon on his left arm, and it is obviously much smaller than Megatron's (and most likely far less powerful).
Megatron's iconic cannon served admirably as a cutting torch when Megatron sought to gain access to the Autobots' ship.  . After Megatron's revival his newly rebuilt fusion cannon proved even more effective against the Autobot ship, knocking it clean out of the sky in under a minute with some help from Lugnut and Blitzwing. It also has a lense. sometimes. It was last seen repeatedly killing the newly-immortal Starscream. It finally blasted an Autobot (SERIOUS) but was later smashed into pieces by the said Autobot (also SERIOUS).
Notably Megatron has only fired four shots directly at the Autobots, missing each time. The writers are likely reluctant to have half the cast atomized in the space of five seconds.
War for Cybertron
Megatron has a fusion cannon and any squishy who selects him in the four missions he's a playable character in can use it. It's a very powerful weapon that does a lot of damage, but has fairly limited ammunition versus some of the chain guns in the game. In tank mode, however, while visually it appears to be the same weapon, it has even less ammunition.
Oddly enough, the Fusion Cannon is indeed available for use in multiplayer, but only with the Leader class which Megatron is not a part of, instead being classified as a Soldier; thus, he cannot use his signature weapon in that game mode. However, Optimus and Soundwave can since they are part of the Leader class. What the freaking slag?!
- The deep percussive note at the beginning of the iconic fusion cannon sound effect (used throughout G1 and Animated) is an old "stock" sound used frequently in American animation, most notably within Hanna-Barbera's "Scooby Doo" where it was used to signify cast members or objects falling on the ground, running into walls, etc.
- Megatron's weapon has been renamed the Riot Cannon in Fall of Cybertron.
- ↑ Use of sound-effects from the original cartoon only served to enhance the weapon's coolness.