GAU-53 70mm rotary cannon
From Halopedia, the Halo wiki
The GAU/53 is a triple-barrelled rotary cannon, capable of firing a fully-automatic barrage of 70mm ammunition.
Two of these autocannons are mounted below the nose of the D79-TC Pelican and its gunship variant, and the D81WP Condor. However, unlike the standard class, the Pelican gunship variant is only armed with one mounted autocannon. The autocannons are mounted on a gimble on the Pelicans to allow them to rotate with ease.
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