Fusion cannon

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'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.

Fiction

Generation 1

Marvel Comics continuity

Hey-o!

Presumably, the fusion 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.

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.

Generation 1 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. Fire on the Mountain

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. More than Meets the Eye, Part 3 Starscream's Brigade

IDW comics continuity

The fusion cannon was a gift from Starscream. Megatron Origin issue 4 It's ironic how Megatron decided to pay him back a few hundred thousand years later by blowing a huge hole in his torso. Infiltration issue 6

Beast Wars cartoon

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

Although not Megatron's, according to Beast Wars, the Ark's cannon was also a fusion cannon. The Agenda (Part III)

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 is presumably telescopic, as he keeps it stored in his left arm when not in use. Its powerful blast is enough to knock Optimus Prime back three city blocks and requires Megatron to use both arms to steady the aim. Because Megatron is always concerned about collateral damage. Transformers (2007)

It is also mentioned on Ironhide's toy 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.

Animated cartoon

Given time, Starscream may simply develop an immunity to the cannon.

Megatron's iconic cannon served admirably as a cutting torch when Megatron sought to gain access to the Autobots' ship. Transform and Roll Out 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 lens... sometimes. Megatron Rising - Part 2 It was instrumental in repeatedly killing the newly-immortal Starscream, Mission Accomplished and used in the capture of Bulkhead. A Bridge Too Close, Part I

Eventually, it (as well as his swords; this was not seen, but his "weapons", plural, were referred to) was removed when he was captured by Omega Supreme's internal defenses. Optimus Prime discovered, painfully, that Megatron was not as weak without it as Optimus had hoped. TransWarped

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.

Notes

• 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.