M68 Gauss cannon
This article is about the model introduced in Halo 2. For the model introduced in Halo 5: Guardians, see M555 Gauss turret.
The M68 Asynchronous Linear-Induction Motor, more commonly known as the M68 Gauss Cannon (named after nineteenth century physicist Carl F. Gauss), is a weapon that fires hyper-velocity, high-density projectiles similar to those of a Magnetic Accelerator Cannon, except on a much smaller scale.[note 1] Also known among the troops as the "68," "Rail Gun," "Tent Pole," and the "Gas Gun," it can be mounted on a stationary stand, or on the rear of the M12G1 Warthog LAAV.
The M68 ALIM uses an asynchronous linear-induction motor to produce a bipolar magnetic field capable of launching a 25x130mm projectile at an incredible speed of just under Mach 40, or approximately 13.7km per second.[note 2] The great velocity of the projectile is the key to the stopping power and performance of the Gauss cannon, giving it exceptional armor penetration at impressive ranges.
The M68 ALIM is primarily used in an anti-vehicle capacity by UNSCDF ground forces, though it can be used to devastating effect against infantry. It is often seen mounted on the Warthog in the same manner as the M41 LAAG, and while it does not possess the LAAG's extreme rate of fire, it displays near-pinpoint accuracy and overwhelming firepower, usually killing infantry targets with a single shot, and heavily damaging those in the immediate vicinity of where the round impacts. Its intended usage is similar to that of a M39 rocket turret, though the rocket turret does not possess the all-around utility and adaptability of the M68 ALIM and causes much more collateral damage.
The M68 is an extremely accurate weapon, useful against most enemy infantry and vehicle units. Unsurprisingly it has proven to be devastating against light-armored vehicles; it can take out a Type-32 Ghost or a Banshee in one or two shots (depending on shot placement). The weapon is 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. Firing at the vehicle's armored front is less effective, but the rate at which the weapon cycles can compensate for this. The M68 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 before moving onto smaller, less dangerous targets. Its relatively high rate of fire, when compared to other electromagnetic accelerator weapons, makes it a devastating precision weapon on any battlefield. Its anti-armor capabilities are second only to those of the SP42 Cobra's mounted M66 light rail gun, and static defense platforms, such as the Onager, and its portability more than makes up for this.
The M68 has few disadvantages, mainly the fact that the weapon (like all other vehicle-mounted turrets) offers little to no protection to the operator, and allows them to be easily sniped without the presence of a skilled driver. It lacks precision at extreme distances, but can still hit large targets (i.e. Wraiths and Phantoms) with a fair degree of accuracy. The Gauss Cannon can also cause permanent hearing damage when fired within 20 meters of unprotected infantry. The M68's muzzle flash can also give away its user's position. 
List of appearances