It looks like a surface to air missile launcher (a big one, but still). What does NULL have to do with it, and why is it referred to as a Cannon?

Noisy
Unusually
Large
Lemmings

FTFY

The Mk-13b ADMS (nicknamed the NULL Cannon due to a database glitch in the inventory system when it was first deployed) is a medium range air defense missile system designed to protect an entire planetary district from in-atmosphere attack.

The Mk-13b is an extremely effective system. For the most part, it renders a district completely invulnerable to airmobile ground attack. It is particularly effective at eliminating Mobile-Command class units in the upper atmosphere.
Recently, however, a vulnerability in the system has emerged. It appears that it is possible, using a brute-force method, to hack the IFF codes used to authenticate commands to alter engagement rules. While it was known that this was possible when they were first deployed, the facts that, firstly, any such hacking attempt would take a minimum of 24 hours and, secondly, that the attempt would alert the owner of the district of the impending attack was assumed to be sufficient to make this method of attack futile.

Further, in order to change the IFF codes once the hacking attempt has started, an MCC class vessel must be deployed within 2 km of the array during the 1 hour window in each 24 during which the MK-13b will accept commands once the hacking attempt is detected. Again, it was assumed that requiring the deployment such a powerful ground asset would limit the utility of the hacking attempt.

However, it was discovered that if two MCC class vessels both attempt to access the Mk-13b array during this period, the system reverts to local control. Each missile launcher must then be manually accessed and reset to define its engagement parameters. It is this fact that has allowed the Mk-13b system to be circumvented.

This has led to peculiarly formalized battles between the immortal mercenaries fighting for control of planetary districts, particularly in Molden Heath. The attacker begins the hack attempt, effectively announcing that he will attack that district during the next access period after the 24 hours necessary to break the IFF codes. Both attacker and defender deploy MCCs to force the MK-13bs into local control and then fight for control over them until one of the MCCs is destroyed, at which point the system reverts to centralized control. Interestingly, even if the attacker wins the field, the system will prompt central district command for the override, meaning that as long as at least one cloned soldier remains in the district, it will not switch control to the attackers, and the attacker must begin the process again. In Molden Heath, where districts stockpile clone as a strategic asset, this has led to drawn out formalized battles for district control that last only minutes but that span several days. In other areas, all the clones in the district are housed in the MCC, so its destruction is sufficient to ensure victory.

It looks like a surface to air missile launcher (a big one, but still). What does NULL have to do with it, and why is it referred to as a Cannon?

dunno but they really hurt if you manage to get in the way of the missiles.

dunno but they really hurt if you manage to get in the way of the missiles.

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What i'd like to know is what form of sorcery is it that causes missiles to turn 90 degrees instead of hit the MCC from above via a parabolic arc?

If the Null Cannons are supposed to be the only weapons in Dust that can hurt an MCC why do the MMC's shoot each other, and actually do damage

Presumably it was originally supposed to shoot Null blaster ammo, which based on its size would probably make it ~ a small sized turret in eve.

I think they do come down from above. The Null Cannon Missiles go straight up and disappear, then reappear directly over the MCC and hit it. The missiles from MCC to MCC are the ones you've got to dodge.