From Halopedia, the Halo wiki
Looking for the closely related Covenant vehicle weapon, the fuel rod cannon, or for the handheld fuel rod gun?
The assault cannon is a powerful weapon used by the Covenant. Similar to the fuel rod gun, it is most commonly utilized by Mgalekgolo, although the Hierarchs' anti-gravity chairs are equipped with a variant of the weapon. The weapon has two primary firing modes, one of which fires the incendiary gel in globs similar to the standard fuel rod gun while the other fires a continuous stream of the gel.
The standard version of the weapon is incredibly heavy, making it suitable only for Mgalekgolo. The weapon is integrated into the right "arm" of the Mgalekgolo's armor. In most models, the nozzle is surrounded by three metal "fingers", which may serve as manipulators or assist in the guiding of the projectile. Several translucent tubes project from the cannon, containing the glowing green fuel used in the weapon's firing. The Hierarchs' gravity thrones are also equipped with a form of the weapon that is fully integrated to the throne's armrests.
The weapon uses tubes of radioactive incendiary gel, very similar to those fired by the standard fuel rod gun, as ammunition. The gel can be fired in globs, as seen in Halo: Combat Evolved, Halo Wars, Halo: Reach, and Halo 3: ODST, which detonate on impact and have an arcing trajectory. The weapon can also fire the gel in a "beam", as seen in Halo 2, Halo 3, Halo Wars, and Halo 3: ODST. In both cases, the weapon must charge before firing. The sound and light emitted during charging provides early warning to opponents that the Mgalekgolo is about to fire.
The gel used in the Mgalekgolo's assault cannons is exclusively radioactive green in color, while that used by the Hierarchs' gravity thrones is golden yellow.
Variations between the games
In terms of gameplay, the assault cannon has changed with each installment, in terms of operation and aesthetics, mostly to balance gameplay.
List of appearances