M810 Helix point defense gun
From Halopedia, the Halo wiki
The M810 Helix point defense gun, also known as AAA Helix gun or Helix System, is a United Nations Space Command starship-mounted point defense weapon. This autocannon can be found affixed to human capital ships during the Insurrection and the Human-Covenant War as late as the Battle of Psi Serpentis in 2543.
The AAA Helix system worked as a point-defense weapon. If provided sufficient range, Helix guns were capable of intercepting incoming missiles and, presumably, boarding and fighter craft. Their description as being "spun up and hot", suggests that they were some kind of gatling gun. They were likely computer or AI-operated, as with virtually all other ship-mounted autocannons.
List of appearances