The 120mm ventral gun is a weapon mounted in pairs to the underside of the UNSC GA-TL1 Longsword-class Interceptor. It possesses at least a 90 degree swivel, and is able to be retracted into the hull. The weapon's shells are of a higher caliber than the Longsword's main weapon, the 110mm Rotary cannon, and presumably do more damage.
During the capture of the Ascendant Justice by UNSC survivors of the Battle of Installation 04, a Longsword's 120mm Ventral guns were used to clear one of the vessel's hangars of a "legion" of Unggoy. The shells cut through unshielded fighters, fuel pods, and the Covenant defenders with ease.