From Halopedia, the Halo wiki
The M370 autocannon, also known as the "Pelican Gun" or "Nose Gun," is an anti-personnel/anti-vehicle weapon mounted below the nose of most D77-TC Pelican dropships. The weapon fires 70mm depleted uranium slugs and is sometimes twin-linked with another such weapon, doubling its firepower. The gun is linked to the movements of the pilot's head, which allows for very short delays between spotting enemies and firing; the turret can rotate up to 120 degrees to wherever the pilot is looking. The turret can also be operated by a copilot, or manually by a gunner. Although powerful, it is not very accurate, and troops must rely on its fully-automatic firing to make cover. In space, or other significant distances, the Pelican must rely on its speed and maneuverability for protection. The weapon seemingly replaced the 40mm chain gun that was common prior to 2525.
Pilots and gunners of D77-TC Pelicans equipped with M370s are able to use the Integrated Helmet and Display Sight System to allow them track targets with head movements and display weapon information on their heads-up display.
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