Automated plasma turret
From Halopedia, the Halo wiki
The Automated Plasma Turret is 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 are of Covenant technology and surround the perimeter of the map. 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. They function towards the same end as an invisible wall, to keep players within the believable confines of the map.
So far, it is impossible to get out of the turret's line of fire. The turret's rounds fire through Bubble Shields and Deployable Covers, and will still hurt players when Invincibility is turned on or if they have overshields, a behaviour similar to the Brute Landmine. In terms of story value, these turrets may have been set up by the Covenant to protect the base from intruders.
