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 appear to be 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 are unique, in the way that they fire through Bubble Shields and Deployable Covers. They will also kill you even if you have Invincibility and/or an Overshield. It is possible to stop their fire by spawning Wraiths in front of them, but by the time you get any distance you hit an invisible wall.
