35mm autocannon

The 35mm Autocannon is a Covenant Loyalist vehicularly-mounted weapon mounted on the Brute Chopper.

It is a semi-automatic 35mm cannon.

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.