From Halopedia, the Halo wiki
The 20mm Sawtooth chaingun is a fully automatic machine gun.
Sawtooths are mounted in the beds of Murat gun trucks used by the Gao Republic. On December 14, 2553, a pair of Murats was protecting a convoy carrying President Arlo Casille when it was ambushed by two United Nations Space Command teams working in concert. The Spartan-IIIs, Mark-G313 and Ash-G099 were able to effortlessly subdue the trucks' gunners and take over their Sawtooths.