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Static automatic turret
The Automatic Turret comes in two variants: the tri-barreled gun (that appears in Half-Life only) and a smaller single barreled gun (that appears in both Half-Life and Decay). The Big Automatic Turret makes its only appearance at the entrance of the Lambda Complex. It can be activated by using the computer or the fuse box located in the security checkpoint.
Automatic Turrets are seen in several places within the Black Mesa Research Facility concealed within the ceiling in an armored cupola. They were set possibly to deal with potential escaped specimens, or are perhaps a leftover from the facility's days as a military missile complex. Some deploy automatically when sighting a target, and some are controlled by nearby levers on connected fuse boxes. When a lever is available, the player can use it to activate or deactivate the gun at will. If not, the turret can be disabled with explosives or gunfire.
The Automatic Turrets do not distinguish friends from foes. Therefore, they will attack anything in range, including all Black Mesa employees as well as Xen creatures.
Behind the scenes
Although it's never used in the game, the turrets can be mounted in floors.
Early model from Half-Life Alpha.