35mm autocannon
From Halopedia, the Halo wiki
The 35mm Autocannon is a Covenant Loyalist vehicularly-mounted weapon mounted on the Brute Chopper.
It fires large-caliber 35mm shells, and is self-loading, although it is not fully automatic.
The 35mm Autocannons are the primary offensive asset of Brute Choppers, and each Brute Chopper is mounted with four linked forward-firing 35mm autocannons.
It is strange that Brutes would use what you could call 'nearly human' ammunition.
