Gauss Cannon
From Halopedia, the Halo wiki
The Gauss Cannon is an anti-vehicle weapon usually mounted on a Warthog. It uses burst damage, unlike the continous damage of the chaingun. It is relatively powerful, also being known as the anti-vehicle Warthog. It uses the same Magnetic Accelerator technology as the orbital MAC gun defense system. It is, in essence, a pint sized version. It can kill infantry in one hit, two if they have an Overshield, and can destroy things most effectively. Due to its power, speed and manueverability, it is a good weapon against Wraiths.
In Halo 2, this version of the Warthog seems to have replaced the M12 LAAV version of the Warthog, which featured three Rocket Launchers. That version became problematic due to the prodigous reload time necessary for the launchers.