M68 Gauss cannon
From Halopedia, the Halo wiki
The M68 Gauss Cannon is a really, really big gun that shoots hypervelocity projectiles similar to those of a MAC gun. It is very powerful, and very useful against vehicles. However it is not as effective against infantry as its cousin, the regular version Warthog.
This Vehicle made its debut in Halo 2, and it is unknown whether it will return for Halo 3.