35mm auto-cannons
From Halopedia, the Halo wiki
35mm Auto-cannons
The 35mm Auto-cannons are used as the Brute Chopper's main and only projectile weapon. The projectiles may be made of plasma.
Description
The Auto-cannons are almost rectangular in shape, and are mounted on Brute Choppers. The projectiles look like orange blobs of plasma but this has yet to be answered. When fired, they can cause massive damage to just about anything. They have been known to destroy wraith heavy tanks very easily.