M68 Gauss cannon

The M68 Gauss Cannon is a weapon that fires hypervelocity, high-density projectiles similar to those of a MAC gun. It is mounted on the rear of the Warthog. It is very powerful, and very useful against vehicles. However it is not as effective against infantry as the M41 LAAG which is also mounted on the Warthog. This is because there is little splash damage and its slower rate of fire, but a direct hit will kill most enemies in 1 shot.

This Vehicle made its debut in Halo 2, however, with the footage shown to the EGM people during their stay at Bungie, where Master Chief boards a Gauss Warthog, it can be assumed it will return for Halo 3.

...After a few quick steps forward, the Master Chief hops into the back of a Warthog with a Gauss cannon mounted on the back. There's the familiar whirr of Warthog engines humming to life and the hog spins out from underneath the building's overhang.