M870 Rampart point defense gun
From Halopedia, the Halo wiki
The M870 Rampart point defense gun is a weapon system utilized by warships of the UNSC Navy, typically as part of a ship-wide defensive network.. Each M870 "gun", or "Rampart", is a fully-automated hull-based turret emplacement, typically consisting of a battery of twin- or quad-mounted 50mm rapid-fire coilguns and a targeting scanner. Point defense guns such as these are used for the engagement of incoming threats, including the warding off of strikecraft and the disruption of Plasma torpedoes; each emplacement actively switches between sub-caliber armor-piercing sabots or proximity detonation fragmentation shells based on a threat-analysis of its current target in order to maximise effectiveness.
M870s are most commonly mounted on UNSC frigates, including Charon-class light frigates, Stalwart-class light frigates and Strident-class heavy frigates. However, they are also utilised on Gladius-class heavy corvettes, Halberd-class destroyers, and even Epoch-class heavy carriers, making them a relatively common fixture across the entirety of the UNSC fleet.
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