Automated plasma turret

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 (although much of the Covenant's technology was stolen from the Forerunners) and surround the perimeter of the map.

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. They function towards the same end as an invisible wall, to keep players within the believable confines of the map. So far, the only things that can block a turret's rounds are objects added through forge.

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 behavior 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. They would be effective at mowing through enemy infantry, especially against the Flood.

Activation
The turrets use motion trackers, to know when enemies approach them. These devices are presumably used to prevent intruders to get inside the fortress, as their motion trackers are pointing outwards. In gameplay, the turrets are activated when a player goes outside from the map (and the combat too), as the players respawn inside the detection limits. Strangely the turrets seem to have a main control console inside each of the two bases. One is turned on, but you cannot enter it.

Trivia

 * The Automated Turret may be related to the Covenant Air Artillery. It may also be produced with it as well.
 * The Turrets are completely indestructible, but inventive use of barriers to stop their fire nearly doubles the usable area of Snowbound.
 * Overloading maps with these turrets or make the turrets disappear and do not fire at you, making it easier to glitch out of the maps. After a while the turrets will return and follow you but will not fire.
 * The Auto Turrets will not fire if you overload the map in Forge mode.
 * Because of its purpose, even Bubble Shields will not protect you from Auto Turrets.
 * Strangely enough, if you go to the map in Theater mode, the alarms will still sound if you fly beyond the turrets. Obviously, they do not shoot at you.
 * The turret's ability to shoot through Bubble Shields, and Deployable Cover shouldn't be regarded as canon. It is simply to keep players from leaving Snowbound's play area for too long.