Fusion cannon

The name or term "Cannon" refers to more than one character or idea. For a list of other meanings, see Cannon (disambiguation).

A release of lubricant is quite common when seeing this.

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' hands, then.) Only Optimus Prime appears to be able to survive a shot from the fusion cannon, considering how many times Megatron has shot Prime.

Its unique sound-effect can best be described as "ba-WHOOMPH"

Fiction

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. WTF?

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. Nemesis, Part 2

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.

From Teletraan I: The Transformers Wiki, a Wikia wiki.