The Fusion Cannon is Megatron's ultimate weapon serving as a dominant part of his alternate mode. It is devastatingly powerful as demonstrated by its removal of Ironhide's head among other things. (It's at least as powerful as Fortress Maximus's hands.) It strikes fear into the sparks of both Autobots and Decepticons alike, for any who challenge such a weapon will surely fail.
The cannon is presumably powered by a nuclear reaction. Sometimes Megatron has been able to link to a black hole or antimatter to augment its power and himself, increasing it astronomically and making him look space-rippy in the process. Most bodacious!
Generation One cartoon
The cannon served as the scope of Megatron's weapon mode and could be removed from his body. One time Brawn stole it and used it against him. Megatron was not amused. Megatron presumably increased its power permanently to the point of being able to help destroy a ship full of Autobots including Ironhide. Unfortunately, he lost it during a desperate battle with Optimus Prime and had it replaced by Unicron with a particle cannon.
Fusion cannons are also useful when you just wanna hit Starscream for shooting you but don't want to have to walk over to him... yeah, we don't question that.
The cannon functions as the weapon of Megatron's vehicle mode.
Generation 2 / Wings Universe
As part of an agreement with Cobra, Megatron was rebuilt to resemble a human tank with his cannon being replaced by a railgun.
Beast Wars: Transformers / Beast Machines
Megatron loved his cannon so much, he had another one installed on the Nemesis. The blast radius from the weapon is massive. Most kickass. It requires a recharge time to fire again. Less kickass.
The fusion cannon was a gift from Starscream.How ironic that Megatron decided to pay him back a few hundred thousand years later by blowing a hole in his torso. Galvatron also had a fusion cannon as well.
Megatron once had the cannon replaced with a railgun and destroyed it when he renounced the Decepticon cause; he would later receive a replacement made from Mortilus's teleport chamber which was destroyed in a desperate battle against the Decepticon Justice Division, though it was returned to its original function later.
Tarn, leader of the Justice Division, wielded a double fusion cannon.
Live-action film series
Megatron's cannon is presumably telescopic as he keeps it retracted in his left arm when not in use. However, he did fire a blast from his right arm that sent Jazz flying several feet, so it may be possible that each arm stores a fusion cannon and they combine for an increase in power. Its blast is powerful 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 his aim because he is somehow concerned about collateral damage (or it could be so powerful that without balance he could knock himself back a slight distance).
After returning from the dead, Megatron had the cannon stored in his right arm. The cannon was shorter and larger than before and also had a bayonet-like spear called the death-lock pincer attached to its underbelly. Its most notable uses were during his battle against Optimus where he impaled and blasted the Autobot leader and for killing Sam Witwicky. Later, while battling Jetpower Optimus Prime, Optimus used Megatron's cannon to blow off half his face before slicing off his right arm along with the cannon and blasting him through a wall.
Megatron's cannon once again changed forms along with Megatron himself. It was no longer attached to his arm and used more like a conventional weapon; this allowed for others to use it. Megatron carried it on his back when not in use and it now resembled a sawed-off stockless shotgun. Megatron used it to incapacitate Sentinel Prime and save Optimus, dropping it when the latter killed him. Optimus then picked up the cannon and blasted Sentinel, discarding it down once the deed was done. Optimus kept this cannon while on the run to replace his ion blaster but lost it upon being captured by Lockdown.
It is mentioned on Ironhide's packaging that he also has a fusion cannon; it is presumably the weapon on his left arm, and is obviously much smaller than Megatron's (and likely less powerful).
Megatron once again had the cannon attached to his body while in the service of Quintessa; aside from firing energy bolts it could also function as a flamethrower. Unfortunately, during his final showdown with Optimus and the other Autobots his eternal foe sliced off his arm and the cannon... again.
Megatron's fusion cannon was mounted on the right arm of his robot mode and front of his vehicle mode; it served admirably as a cutting torch when he sought to gain access to an inactive Omega Supreme and retrieve the AllSpark. After Megatron's revival the rebuilt cannon proved even more effective against the deactivated ship-bot, knocking it clean out of the sky in under a minute with help from Lugnut and Blitzwing. It also has a targeting lens sometimes. It was seen repeatedly killing the newly-immortal Starscream, being used in the capture of Bulkhead and confiscated along with Megatron's other weapons by a reactivated Omega but later recovered and used to cripple Starscream once more, though not before Optimus Prime had learned painfully that Megatron was not as weak without it as expected. It finally blasted Optimus once or twice though both shots were ineffective. It even survived a detonating Lugnut Supreme along with the rest of Megatron but was ultimately smashed into pieces with the Magnus Hammer.
Notably, Megatron fired multiple shots directly at the Autobots, missing most of the time. The writers were likely reluctant to have most of the cast atomized in a short duration. However, to be fair only Starscream was close enough at point-blank range to be hit and the only times it connected with someone else it was presumably not fully-charged.
Megatron has a fusion cannon and any organic who selects him in the playable missions can use it. It's a very powerful weapon that does a lot of damage but has fairly limited ammunition compared to most long-range weapons. However, while it visually appears to be the same weapon in vehicle mode its ammunition is reduced.
Oddly enough, the cannon is indeed available in multiplayer only with the Leader class which Megatron is not a part of; thus he cannot use his signature weapon in that game mode. However, Optimus Prime and Soundwave can since they are part of the Leader class. What the slag?!
The cannon is Megatron's signature weapon attached to his right arm capable of doing more damage than Energon or ion blasters. It presumably may have had something to do with the twin lasers of his vehicle mode or they likely shared the same power source. In Megatron's case, it appeared to fire Energon blasts enhanced by Dark Energon which could temporarily disable Transformers but severely cripple humans. It also had a retractable blade as well.
Megatron abandoned his cannon when he acquired Liege Maximo's arm to replace his own in order to activate the Forge of Solus Prime allowing him to create the Dark Star Saber. However, he later lost that arm in the fight for the Omega Lock and regained his original arm along with the cannon.
Megatron's fortress Darkmount on Earth was armed with fusion cannons which functioned differently from the one on his arm; they instead gathered and compressed objects in an energy field which was released in an explosive burst. When human forces arrived to wage a counterattack Megatron had one of the cannons fired at them, annihilating the military and destroying Jasper in the process. Megatron warned William Fowler that further human interference would result in the cannons being turned on major settlements.
When Ultra Magnus arrived and reunited the majority of Team Prime, he and Fowler resolved to disable the cannons before destroying Darkmount with the help of Earth's military. While Bulkhead and Wheeljack created a distraction at the base of the fortress, Arcee and Bumblebee attempted to sneak into the fortress to disable the cannons, but when met with heavy resistance Magnus took matters into his own servos and mounted a one-bot assault. He was stopped by Megatron and though the Autobots were all captured, a rebuilt Optimus Prime's arrival quickly resulted in the cannons being crippled by a fight between him and Megatron and destroyed by Skystrikers. Trypticon as the Nemesis also possessed fusion cannons.
After being killed by Bumblebee, he lost it upon being resurrected and possessed by Unicron though he was able to access two Dark Energon cannons that were extremely effective.
Megatronus had a fusion cannon of his own once, which led to a reformed Nexus Prime taking notice of the similarities between him and the Decepticon leader. Upon being transported to Earth by Steeljaw's Pack, he had two fusion cannons that could fire blasts of purple energy strong enough to knock a Thirteen-empowered Optimus backwards; in vehicle mode the cannons functioned as a single turret.
- The deep percussive note at the beginning of the iconic cannon's iconic sound effect used in G1 and Animated is an old stock sound effect used frequently in American animation, most notably Scooby Doo where it was used to signify cast members or objects falling on the ground, running into walls, etc.
- It was renamed the Riot Cannon in Fall of Cybertron.
- Use of sound-effects from the original cartoon only served to enhance its coolness.