M512 smooth-bore high-velocity cannon

The M512 smooth-bore high-velocity cannon, or simply the M512 cannon, is a 90mm gun mounted on M808 Scorpion tanks.

Design details
The M512 cannon is designated to 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.

Ammunition
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.

Due to the similar in-game performance of this round in Halo 3 compared to the ammunition featured in other Halo games, it appears that this tungsten projectile is the primary variant of ammunition utilised by all Scorpion tanks seen across all Halo games, with the exception of the M820 Scorpion in Halo 5 Guardians which utilises the different M990 electrothermal-chemical smoothbore cannon. Due to the use of tungsten in its construction, it is possible that this ammunition is some form of kinetic energy penetrator round - this is supported by the fact that the M512 cannon is smooth-bore, which is more suited for such ammunition. The counter-points to this are twofold. Firstly, the Marine who refers to the M512 cannon as firing tungsten ammunition states; "how does 90 millimetres of tungsten strike you?" - realistically, the tungsten penetrator of such a round would encapsulate far less than the diameter of the full round itself, though this could be disregarded as simply inaccurate speaking on his part for the sake of taunting. The second counter-point is that the round exhibits powerful explosive properties in-game, which a kinetic energy penetrator typically would not. This could be explained as a full-gameplay concession and thus non-canon, a partial-gameplay concession with the Scorpion's turret selecting rounds with explosive properties to engage un-armored targets with only the explosive effect on enemy armored vehicles being non-canon, or as a follow-on explosive charge working alongside the sabot or penetrator to give it more tactical flexibility and lethality.

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

Advantages
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.

Disadvantages
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.

Campaign

 * In Halo: Combat Evolved, the cannon is quite inaccurate; the shot had high deviation, but compensated with a large explosive radius.
 * Using the M512 in the level Delta Halo in Halo 2 is tricky, particularly when the player moves into tight quarters and encounters Type-32 Ghosts due to the steep angles of the Forerunner structures. Self-harm is a constant worry. It is best for players to remain in an open spot and let the Ghosts come to the player, picking them as soon as they appear. If this option is not available it is recommended that players use the Scorpion's turret machine gun instead, which is just as effective thanks to its impressive accuracy and high damage.
 * If the player acquires a Scorpion near the beginning of Quarantine Zone, the rest of the mission becomes spectacularly easy; the range of the M512 allows the user to pulverize any and all Flood resistance in their way, and two-shot Sentinel Enforcers with ease.
 * The M512 is undoubtedly at its best in Halo 3; its accuracy is near pin-point with almost no drop, and the splash damage is predictable enough to allow for a skilled tank gunner to eliminate entire Covenant squads with a single round. This is most visible during the latter part of The Ark, where engagements with enemies take place across wide open areas
 * In Halo 3 on the level The Ark, Phantoms can be brought down with a few well placed shots at one of their engines.

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 AV-14 Hornet or the Type-26 Banshee, players must also aim ahead of their target as well as above to ensure a direct strike. Depending on how fast the target is moving, the reticule needs to be slightly ahead of the target or just outside the reticule.
 * In Halo: Reach, most maps that spawn a Scorpion have large, open lines of sight; perfect for the cannon's accuracy. Sit in relative safety and pick off targets of opportunity.
 * In Halo 4, the cannon's shots produces a noticeable vapor trail similar to that of a sniper rifle when fired, and reveals the tank's location. The cannon's shells travel much slower than they did in Halo: Reach, and arc at a higher rate than before. Also, unlike previous Halo titles, the tank round can be bounced off some surfaces.

Trivia

 * If the cannon is modded onto another vehicle, such as a Warthog or a Hornet, the weapon's recoil will be so powerful that the smaller vehicle will go flying.
 * "High-velocity" is a real-world term for a tank cannon with a projectile velocity between 1,550 and 3,350 feet per second (472 m/s to 1,021 m/s), such as the 90mm M3 Gun used by the M26 Pershing.
 * In Halo: Reach, it is possible to shoot off the cannon of the Scorpion using another Scorpion.

List of Appearances

 * Halo: The Fall of Reach
 * Halo: Combat Evolved
 * Halo 2
 * Halo 3
 * Halo Wars
 * Halo 3: ODST
 * Halo: Reach
 * Halo: Combat Evolved Anniversary
 * Halo 4
 * Spartan Ops
 * Halo: Spartan Assault
 * Halo: Escalation
 * Halo 2: Anniversary
 * Halo: Spartan Strike
 * Halo: Ground Command