M512 smooth-bore high-velocity cannon
From Halopedia, the Halo wiki
The M512 smooth-bore high-velocity cannon, or simply the M512 cannon, is a 90mm gun mounted on M808 Scorpion tanks.
The M512 cannon is stated to have a smooth-bore high-velocity construction. Smooth-bore indicates that the barrel of the gun lacks rifling, and high-velocity indicates that the cannon is engineered for the firing of ammunition with a notably high muzzle velocity. The cannon is typically mounted in the un-manned turret of an M808-series Scorpion tank and fed via autoloader.
The M512 cannon is compatible with at least two varieties of ammunition. The first is a projectile which includes tungsten in its construction, and was utilised by Marine M808C tank units attached to the Forward Unto Dawn during the Battle of the Ark. The second type of ammunition employed is the S1 canister shell, which provides the Scorpion tank with a powerful method of engaging large numbers of lightly-armoured targets, being especially lethal against infantry 
The M512 cannon can effectively destroy most light vehicles with only one shot. Even heavy vehicles like the Covenant Type-26 Wraith can only withstand two to 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 Mgalekgolo in just one shot. Shots from a 90mm Cannon are also very effective at destroying walls or structures if enough shots are fired at it. The cannon also has very little recoil, which may be attributed to an added muzzle brake or its design.
The M512 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 cannon's 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 M512 cannon must be reloaded after every shot and takes between three and four seconds, which may give the enemy enough time to react against the tank. In addition, the smoke trail the round leaves behind when fired can compromise the tank's location, making it easier for enemies with anti-armor weaponry to locate the Scorpion in multiplayer.
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