Fusion Cannon

The Fusion Cannon is a large arm mounted cannon used by almost every incarnation of Megatron from the Transformers franchise.

Generation One
Being the most iconic incarnation of the tyrant wielded weapon, the Fusion Cannon is a large black rifle mounted to Megatron's forearm that fires huge blasts of purple energy. Ironically, despite being a huge destructive firearm, it is simply Megatron's scope when he transforms into his gun mode. Galvatron, a rebuilt version of Megatron, wields a similar cannon. It is swirled, slick, and colored orange, and is the primary cannon in Galvatron's cybertronian howitzer mode.

Live-Action film series
In the first film, the cannon is extremely different from its original counterpart. Instead, it is formed when Megatron merges his hands together to forge one large rifle. He then draws gamma-irradiated plasma from the cosmos before charging it up into one big blast and firing it at his unfortunate opponent.

In the second film, fitting with Megatron's resurrection and redesign, the cannon is much different than its other incarnations. Now it is a thick broad cannon that replaces Megatron's forearm and also has a large metallic lobster-like claw merged into it called the Death Lock Pincer, making it a combination of both a cannon and a sword.