Automated plasma turret
From Halopedia, the Halo wiki
An automatic turret used to stop players from going out of Halo 3 multiplayer levels. It is not in campaign. The only level that has them is Snowbound, although Sandtrap has a minefield that works the same way.. 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.
