From Halopedia, the Halo wiki
A point-defense gun is a type of United Nations Space Command starship-mounted magnetic accelerator weapon. Though several models exist, the most common type of point-defense gun fires 50mm high-explosive projectiles.
The point-defense gun fills a role similar to the CIWS designs created during the 20th century. Based around magnetic linear accelerator technology, it fires solid projectiles. Unlike Archer missiles and the MAC weapon systems mounted on capital ships, the point-defense guns are not used to engage Covenant capital ships. They are instead meant to intercept smaller threats like Type-31 Seraph fighters or boarding craft, and disrupt incoming plasma weaponry, to reduce the damage inflicted to a warship's superstructure.
Because human operators lack the reflexive responses necessary to operate the point-defense guns to maximum effect, the autocannons are computer controlled and, if an AI is available, control is diverted to it. "Smart" AIs seem better able to operate the point-defense guns than standard computers and also lesser "dumb" AIs. Nonetheless, in times of need, human operators are capable of manning the turrets when an AI is unable to.
Point-defense guns have been standard armament on UNSC vessels since the Insurrection, a common model being the M810 Helix. Later, Halcyon-class cruisers used a dual-barreled point-defense weapon system designated as the M910 Rampart point defense network.
During the opening stages of the Battle of Installation 04, the ODSTs of Fireteam Raven were instructed by Wellsley to man the UNSC Pillar of Autumn's defensive guns to fend off several waves of Seraph and Banshee fighters. During the later human occupation of Installation 04, the ODSTs that recaptured and secured the wreckage of the UNSC Pillar of Autumn from the Covenant managed to scavenge several of the ship's 50mm turrets. The ODSTs utilized the scavenged defense weapons and used them as anti-air defensive emplacements at Alpha Base.
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