The Super-Graviton Cannon is a weapon that uses gravitational waves condensed into a narrow beam to deal damage to the target. The effective range of the Super-Graviton Cannon is limited only be the vessel's engine power. Normally it is used at the distances of about 10 to 20 kilometers, however I-401 used her (originally, battleship Hyuuga's) cannon to snipe an enemy submarine from about 100 kilometers.
There is no known material can be used to create an armor that can protect something from the shot of a Super-Graviton Cannon. Even the Wave-Force armor of the "Fleet of Fog" vessels is unlikely to protect the said vessel from more that one direct hit. In many cases, weaker Wave-Force armors are completely useless - the resupply vessel Milchkuh was taken out in one shot even though she had her Wave-Force armor at full power. The Mirror-Ring system seems to be the only definite way to protect oneself from a Super-Graviton Cannon shot.
While the Super-Graviton Cannon is a very powerful weapon, it requires a great deal of power and some time to charge up. It's not easy to re-aim it, which means that it is not effective against targets that can move. Additionally, in order to fire the Super-Graviton Cannon, a "Fleet of Fog" vessel needs to open the Klein field of her Wave-Force armor in the direction of the shot, leaving it completely vulnerable to the attacks coming from that direction. During the fight in Yokosuka bay this weakness was used to sink two battleships (Kirishima and Haruna) with a single Corrosive Warhead Torpedo.
Another weakness of the Super-Graviton Cannon is how noticeable its usage is: not only the cannon's charge up creates an anomaly in gravitational field - which can be easily registered even with human technology - but the said anomaly can even be seen with a naked eye. It appears as a distortion in space that varies from a small sag for heavy cruisers to ocean parting all the way to the bottom for heavy battleships... Though, if you see one, it is likely that it is too late to do anything. Firing the cannon also causes lingering disturbances in the spatial phase of the area, which will cripple even the Fleet of Fog's communications if multiple shots are fired.
Normal Super-Graviton CannonEditAll "Fleet of Fog" vessels starting with heavy cruisers have access to Super-Graviton Cannons. Though Takao explicitly states that only battleships and above can have these cannons that can cause significant distortions in space.
A normal Super-Graviton Cannon consists of several gravity-focusing units that are levitated near the vessel during the cannon's use. Heavy cruisers like Takao appear to have only two such units. Battleships like Haruna or Hyuuga are equipped with at least four. Superbattleships Yamato and Musashi have no less than twenty, they don't need the Gravity-Focusing Pods that heavy cruiser and normal Battleships need and the superstructure don't need to be modified to support it; and they are the only vessels who displayed the ability to fire several independent blasts simultaneously.
Under normal conditions, a submarine like I-401 can't have her own Super-Graviton Cannon, but after I-401 sunk battleship Hyuuga, Hyuuga's cannon was looted and later installed inboard of I-401. While it looks like all four gravity-focusers of Hyuuga's Super-Graviton Cannon are used by I-401, all of them have been modified to better fit in the small space available inboard of a submarine.