Halo: Assault Cannon

The Assault Cannon^[1]^[2] 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 weapon, the other of which is closer to the vehicle variant and fires a continuous beam.

Design Details

The weapon is incredibly heavy, making it suitable only for the Hunters, as no other races could carry such a massive load, showing the immense strength of the Hunter. 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 that limb. This makes the weapon impossible to be used by the player in any game. It is unknown how the weapon is fired.

Ammunition

The weapon uses tubes of radioactive incendiary gel, very similar to those fired by the standard fuel rod gun, as ammunition.^[1] The gel can be fired in globs, as seen in Halo: Combat Evolved, Halo Wars, Halo: Reach, and occasionally in 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. 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 very obvious that the Hunter is attempting to shoot an enemy, and it also makes it fairly easy for players to dodge the cannon's blast.

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.
In Halo: Combat Evolved, it was only able to fire single shots that were similar to fuel rod gun 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. Because it is a continuous beam this makes it possible to survive a hit from it on Legendary difficulty by only being hit by the beam for a small amount of time.
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 movable cover, such as supply crates. They are also still extremely easy to avoid.
In Halo 3: ODST, the Hunters appeared in two different forms in two different colors. This first is the traditional blue-armored Hunter, armed with an assault cannon that fires a continuous stream of radioactive incendiary gel. The other wears gold-colored armor and is armed with an assault cannon that fires a single bolt of radioactive incendiary gel which is similar to the weapon model featured in Halo: Combat Evolved.
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 beam version for extra damage.^[3]
Halo: Reach features a full return to the Halo: CE -era assault cannon, which fires single blobs of bright green gel much like a fuel rod gun.

Trivia

When using the Bump Possession cheat in Halo: Combat Evolved to control a Hunter, holding left click down and repeatedly right clicking allows the controlled Hunter to rapidly fire its Assault Cannon without overheating. Whether this accelerated manner of attack is actually utilized at all in AI controlled opponents' behavioral patterns is unknown.
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 said cannon. The player can use this to his/her advantage to avoid being injured by standing completely still and firing from that position.
In Halo 3, green electricity protrudes from the three parts jutting out of the weapon and they form together to make the beam of energy when the weapon is charging. It is also possible this would create a type of small plasma mortar, and that something in the back of the weapon produces a type of gas to project it, similar to how a flamethrower works where a small flame is projected by gas.
Some of the novels refer to the assault cannon as a fuel rod gun/cannon, while it is officially named in Halo: Contact Harvest.
The same book also references the idea that the Assault Cannon's firing types are settings, both to be used from the same cannon. This means that the difference between Cannons might just be superficial.