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 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. This cannon is surprisingly powerful, its ammunition is probably a larger form of the Spiker round. While it may seem surprising that Brutes would use such human-like ammunition, remember that Brute weaponry is not that different from humans firearms in that they both fire projectiles, not plasma rounds

It most likely uses a Gast mechanism Gast gun.