A turret used to stop players from going out of Halo 3 multiplayer levels. If it is used in campaign is unknown
Thus far (as of December 15,2006) the only level know to contain them is Snowbound. They appear to be of Covenant technolodgy and surround the perimeter of the level. Whenever a player attempts to get outside the level, they are gunned down by the automated turrets.
It is also still unknown why these types of turrets haven't been encountered yet. The most likely explenation it that they are a new weapon in the Covenant armory.