M68 Gauss cannon
From Halopedia, the Halo wiki
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, and it is unknown whether it will return for Halo 3.
