Assault cannon

The Assault Cannon is a variant of the standard Fuel Rod Gun utilized only by the Mgalekgolo. It has two variants, one of which is all but identical to the normal Gun, the other of which is closer to the vehicle variant and fires a continuous beam.

Description
The weapon is incredibly heavy, making it suitable only for the Hunters, as no other races could carry such a massive load. The only known instances of it are fused directly to the Hunter's armor, located on the right "arm" of the colony, built into the Lekgolo that make up the limb. This makes the weapon impossible to be used from the player in any game.

Firing Operation
The ammunition it uses is tubes of incendiary gel, very similar to those fired by the standard Fuel Rod Gun. Tubes can be fired individually, as seen in Halo: Combat Evolved, which detonate on impact and have an arcing trajectory. The weapon can also fire gel or plasma in a 'beam', as seen in Halo 2 and 3. It is presumed that the weapon itself can switch between these two modes by the Mgalekgolo using it. However, before firing, the weapon must be charged up. This "charge" makes it extremely obvious that the Hunter is attempting to shoot you, and it also makes it fairly easy to dodge the cannon's blast in Halo: Combat Evolved.

In terms of gameplay, the Assault Cannon has changed in each version of Halo, in terms of operation and aesthetics, mostly to balance out the gameplay with other weapons.

Variations Between the Halo Games
In Halo: Combat Evolved, it was only able to fire single shots, and appeared to contain its gel supply in a "drum" mounted underneath the arm.

In Halo 2, there were minor changes to its appearance but the weapon fired a continuous stream rather than individual blasts while the "drum" reloaded and fired.

In Halo 3, the Assault Cannon is aesthetically very different, but operates much the same as it does in Halo 2. The cannon in Halo 3 can be cut off instantaneously if the target has gone to cover, which stops it from wasting ammunition. Also, the "claws" seem to have hinges, although they are never seen moving in gameplay. Plus, the beam is a bit faster, a lot more damaging, and can blow away moveable cover, such as supply crates.

In Halo Wars, the Hunters start with the mortar version of the Assault Cannon. Later in the game, the Hunters can be equipped with the Assault Beam for extra damage.

Trivia

 * It is speculated the Scarab's main gun is just an enlarged version of the Assault Cannon due to the similarities of the charge-up and the color of the beam.
 * In Halo: Combat Evolved, Hunters will not continue to track you once the cannon has started to charge, making the Hunters inaccurate with moving targets. Strangely, Hunters will never hit a stationary target meaning that the original Assault Cannon's recoil severely lowers the accuracy of the said cannon. You can use this to your advantage to avoid being injured by standing completely still and firing from that position.
 * In Halo 3, if you look closely at a Hunter's Assault Cannon when it is charging up, green electricity protrudes from the three parts jutting out of the weapon and they form together to make the beam of energy.
 * In Halo 2, the Assault Cannon looks similar to the one from Halo: Combat Evolved, but with the addition of claws on the barrel, making it look a bit more menacing.
 * The Halo 3 variant doesn't contain the "drum" like the previous Assault Cannons. Instead it has 6 tubes on the side of the gun which appear to house the incendiary liquid projectile.
 * Some of the novels refer to the Assault Cannon as a Fuel Rod Gun/Cannon, while it is officially named in Halo: Contact Harvest.