From Halopedia, the Halo wiki
The 35mm autocannons are the primary offensive asset of Jiralhanae Type-25 Choppers, and each Chopper is mounted with four linked forward-firing 35mm autocannons. This cannon is surprisingly powerful, and its ammunition is probably a smaller form of the Type-25 Brute Shot round, fired at a slower rate.
The term "autocannon" is not entirely accurate when referring to the weapon, due to its alien design. However, it is self-loading and fires explosive projectiles larger than 20mm much like its human-made counterparts.
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