Fusion cannon

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 [[Media:Fortmaxgiveshead.jpg|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. (Let's not forget he who took a direct hit from it not two seconds before the screen capture to the right, and lived.)

Generation 1
Presumably, this cannon is powered by some sort of nukular 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.

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...ya, we don't question that.

Generation 2
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.

Beast Wars
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.

Beast Machines
Megatron's personal chariot, Mechatron, comes equipped with a fusion cannon to nullify any accursed Maximal that comes across.

Transformers (2007 Film)
Megatron's cannon in the film is presumably telescopic, as he keeps it stored in his left arm when not in use. Its powerful blast is enough to knock Prime back three city blocks and requires Megatron to use both arms to steady the aim. Because Megatron is always concerned about collateral damage. He also used it in single arm mode to shoot the ground Jazz was standing over, presumably before dragging Jazz off to rip him in half.

In the video games, Megatron's fusion cannon is just his left hand firing a projectile, so it's alot less menacing then in the movie.

IDW
The fusion cannon was a gift from Starscream. How ironic how Megatron decided to pay him back a few hundred years later by blowing him to bits. Concept art for Megatron: Origin also shows that at one point in planning he would have worn a similar-looking jackhammer on his right arm.