35mm autocannon
From Halopedia, the Halo wiki
The 35mm Autocannon is a Covenant Loyalist vehicularly-mounted weapon mounted on the Brute Chopper.
It is a semi-automatic 35mm cannon.
The 35mm Autocannon's are the primary offensive asset of Brute Chopper's, and each Brute Chopper is mounted with four linked forward-firing 35mm autocannon's.
It most likely uses a Gas mechanism.
