M68 Asynchronous Linear-Induction Motor

The M68 ALIM, also known as the M68 Gauss Cannon, is a weapon that fires hyper-velocity, high-density projectiles similar to those of a MAC, except on a much smaller scale. It can be mounted on a stationary stand, or on the rear of the M12G1 Warthog LAAV.

Description
This weapon uses an Asynchronous Linear-Induction Motor to produce a bipolar magnetic field capable of launching a 25mm x 130 projectile at an incredible speed of just under Mach 40, or approximately 13.7km per second. The great velocity of the projectile is the key to the stopping power and performance of the Gauss cannon, giving it exceptional armor penetration. As such, it is a commonly used anti-vehicle weapon of the UNSC Marine Corps. However, the M68 can also be used to eliminate Covenant infantry units with reasonable effectiveness.

It is very powerful and very useful against vehicles. However, it is not as effective against infantry as the M41 LAAG. This is because there is little splash damage and it has a slower rate of fire, but a direct hit will kill most enemies in one shot, or two, if the target has an overshield. Due to its power and reasonable rate of fire it is a good weapon against Wraiths and many other vehicles. Its intended usage is similar to that of the 102mm SC-HE Rocket Turret.

Though thought to be similar to the 16 MJ LRG Rail Gun, the primary armament of the Cobra artillery vehicle seen in Halo Wars, the Gauss Cannon actually functions on a completely different principle. A rail gun simply uses an electrical current to force a magnetic projectile along using the principle of same-pole repulsion; a more complex coilgun, or Gauss gun as it is known in Halo, uses multiple coils to accelerate the projectile through the weapon.

Advantages
The M68 is an extremely accurate weapon, useful against most enemies. Devastating against light vehicles, it can take out a Ghost or a Banshee in one or two shots (depending on shot placement), and it's rather effective against Wraiths as well, provided the driver can out-flank the Wraith and fire upon its unprotected backside before it brings its plasma mortar to bear. The Cannon is also very deadly if used against infantry, however users should aim at the most dangerous units like Sangheili, Jiralhanae and Mgalekgolo in order to make full use of the weapon.

Disadvantages
The M68 has few disadvantages, mainly the weapon's slow fire rate. It is also quite difficult to use at long ranges, especially when shooting at moving targets.

The Gauss Cannon can also cause permanent hearing damage when fired within 20 meters of unprotected infantry.

Tactics

 * The M68 is an excellent anti-vehicle weapon. It will get rid of enemy light armor rather quickly.
 * While not as effective against entire squads, it remains good against single infantry units.
 * Outstanding against Scarabs in Halo 3, since the Warthog on which the weapon is mounted is a smaller, faster target for it to hit. An effective tactic is to remain under the Scarab, avoiding the legs and the main gun, while your Marine gunner targets the Scarab's legs.
 * Experienced gunners using this weapon can effectively use the cannon's extremely high damage and accuracy to 1-shot kill most enemies. This has led to many Gauss gunners being able to attain high multi-kills and killing sprees, especially in multiplayer.

Appearances
In Halo 2, the M12G1 LAAV variant of the Warthog replaces the M12A1 LAAV version of the Warthog, which was featured in Halo PC. The M12A1 LAAV became problematic due to the prodigious reload time necessary for the launchers, and the slow speed of the rockets themselves, although it did not appear in the campaign. A stationary version of the M68 (one of which triggers the flashback level Kizingo Boulevard) appears in Halo 3: ODST. The M68 can also be used near the start of ONI Alpha Site. In Halo Wars, the Gauss Turret is the final upgrade to the Warthog. It allows for much greater offensive and defensive capabilities.

Trivia

 * While operating on a similar principle to the M99 SASR, the shells for the M68 appear to be designed to cause hypervelocity collisions, where the target and slug get almost entirely vaporized on impact. This is probably intended to reduce over-penetration.
 * During the Halo 2 demo trailer, the Gauss fired at a fairly rapid speed. Later, in an early beta multiplayer phase the M68 seemed to be slowed down and also seemed to have lost all its hype. It was later revamped with its original rate of fire during the game's final stages.
 * In Halo 3, the slug in mid-flight is blue with a blue tail, making it look like Covenant weapon's fire at first glance. This 'blue light' is most likely the slug heating the air around it to super heated levels due to friction with the atmosphere. In Halo 2, the trail was yellow-orange.
 * The Halo 3 version of the Gauss Cannon is a bit weaker and has a slower rate of fire. This came in to balance, since the Halo 2-era Gauss Cannon was allegedly too powerful in-game, though it is still devastating in combat.
 * Ferrex said in a thread, "While the Warthog Gauss rifle is modeled as a magnetic acceleration cannon, the sound and effects are actually those of a railgun, which operates on a different principle. However, the sound and effects were so cool that we couldn't not use them."
 * The speed of the Gauss Cannon's projectile in-game is much slower than its canonical speed of Mach 40. This is probably because of design and engine limits. The projectile moves more quickly in Halo 3 than in Halo 2.
 * In Halo 3, the screen on the cannon's firing mechanism will actually display its surroundings in real-time thermal imaging, similar to the scope of the Sniper Rifle System 99D-S2 Anti-Matériel. Unfortunately, this does not have any effect on gameplay, and is only in place for aesthetic reasons.