M512 smooth-bore high-velocity cannon

The 90mm High Velocity Cannon is a United Nations Space Command armament that is mounted on the M808B Main Battle Tank.

Design Details
The 90mm High Velocity Cannon is the primary weapon of the M808B Main Battle Tank, and is mounted on the rear of the tank's chassis. The weapon's design remains for the most part unchanged from 20th century tanks, the 90mm cannon is mounted in a fully traversing turret weapon mount, this allows the cannon to rotate 360 degrees and to move -10 to approximately +60 traverse. A gyroscope is used to stabilize the cannon, allowing it to be effectively aimed and fired at the "short halt" or on the move.

The 90mm High Velocity Cannon is fitted with insulating thermal jackets to reduce gun-barrel warping caused by uneven thermal expansion. Unlike modern tanks however, the 90mm Cannon is mounted on the aft section of the Scorpion's chassis and contains no crew compartment as the loading is done autonomously and the targeting and firing is done electronically from the crew compartment.

Ammunition
The High Velocity Cannon uses a wide variety of 90mm ammunition, however the most commonly employed is the 90mm tungsten Armor Piercing Ballistic Capped Round. This type of round sacrifices armor penetration for larger amounts high explosive filler, this round is accelerated at a high velocity and can strike a target at extreme ranges. Another type of ammunition employed is the S1 Canister Shell, the S1 Canister provides Scorpion equipped tanks with destructive capabilities without the use of traditional high explosives. The Canister round is lethal against enemy infantry when machine guns or other weaponry cannot be used effectively. The S1 is also effective at breaching walls and damaging structures or other vehicles.

Advantages
The 90mm High Velocity Cannon can effectively destroy any light vehicle with only one shot, the cannon has very little recoil and may be attributed to an added muzzle brake or its design, even heavy vehicles like the Covenant Type-25 Assault Gun Carriage can only withstand three rounds before being destroyed. Because of its high area-of-effect and high velocity it is a very useful anti infantry weapon as well, able to kill even the toughest infantry like the Hunter in just one shot. One shot from a 90mm Cannon is also very effective at destroying walls or structures if enough shots are fired at it.

Disadvantages
The 90mm High Velocity Cannon is unable to fire on anything too close to the cannon, or the Scorpion's chassis, as the cannon cannot traverse that close. The cannons high area-of-effect means that precision kills are next to impossible - friendly infantry or vehicles near the target are likely to feel the force of the blast as much as the intended target. The 90mm cannon must be reloaded after every shot and takes between three to four seconds, the time it takes for the cannon to reload may give the enemy enough time to react against the tank; and the smoke trail the round leaves behind when fired, compromises the tanks location making it easy for enemy anti-armor personnel to locate the Scorpion.

Campaign

 * Using the 90mm Cannon the level in Halo 2 will be tricky when the player moves into tight coursers and going up against Ghosts because of the steep angles. It is best for players to remain in an open spot and let the Ghosts come to the player.

Multiplayer

 * In Halo 3, the 90mm Cannon requires players to lead their shot against moving targets move the reticule just slightly ahead of the target and fire. When going up against aircraft like the Hornet or the Banshee, players must also aim ahead of their target. Depending on how fast the target is moving, the reticule needs to be slightly ahead of the target or just outside the reticule.