M910 Rampart point defense network
From Halopedia, the Halo wiki
The M910 PDN is a 50mm machine gun utilized by the UNSC as a point defense system on ships. The UNSC Pillar of Autumn is known to have been equipped with 40 M910 guns.
Sources
http://halo.xbox.com/Content/assets/en-us/ONI/577839090903298928/DDR-A340024-1115-08-27-52.pdf
