Automated plasma turret
From Halopedia, the Halo wiki
An automatic turret used to stop players from going out of Halo 3 multiplayer levels. If it is used in campaign is unknown. The only level confirmed to contain them is Snowbound, although it appears that the level Sandtrap will implement a similar system. They appear to be of Covenant technology and surround the perimeter of the level. Whenever a player attempts to get outside the level, they are gunned down by the automated turrets.[1] It is also still unknown why these types of turrets haven't been encountered yet. The most likely explanation it that they are a new weapon in the Covenant armory. In the first Vidoc a Brute looks seem to be holding a familiar weapon.
