The Aperture Science Sentry Gun is a testing element and a fully autonomous weapon created by Aperture Science and featured in Portal and Portal 2. Sentry Guns are used within Aperture Laboratories to protect restricted areas as well as in the testing of Military Androids and the Aperture Science Handheld Portal Device.
The Aperture Science Sentry Gun has a single red eye, sleek appearance and predominately white color scheme common to most Aperture Science technology. Sentry Guns have only been seen in the Aperture Science Enrichment Center acting as guards in Test Chambers and maintenance areas.
Sentry Guns often speak in a child-like, artificial voice similar to GLaDOS and toilets within Aperture Laboratories. They have a strangely polite demeanor, which is in direct contrast with their deadly behavior. They will even forgive Chell for knocking them over. In one scene in Portal 2, a Sentry Gun stuck inside the Aperture Science Pipe Network attempts to get Chell and Wheatley's attention. At Wheatley's encouragement, Chell ignores it, but the Sentry Gun politely says "thanks anyway."The Sentry Guns are armed with dual machine guns, targeted with a visible laser beam. These weapons are normally hidden inside the body, during "sleep mode." When the Sentry Gun is alerted, they will deploy, and fire at any visible target (usually Test Subjects or intruders in the labs). When no target is in range, the Sentry will scan back and forth across the area, and return to sleep mode after several seconds if no further target is found. Shells for the guns are simply dumped into the hollow body; this allows for an almost unlimited supply of ammunition. Sentry Guns, when idle, conceal their weapons under the two plates on either side of their body. When a gun spots a target it takes a moment to deploy its weapons to fire. A Sentry Gun's weapons are very powerful and are able to kill a human, such as Chell, after a few seconds of continuous fire. The turret may believe it is some sort of dispenser (such as a soda machine), as it says phrases such as 'Preparing to dispense product', 'Sorry, we're closed.', 'Can I help you?' and 'Your business is appreciated'. However, Sentry Guns can be easily spotted by the visible laser beam emitted from their eye at all times, making it easier to avoid them. When knocked over Sentry Guns fire frantically in all directions before permanently deactivating. Like all other objects within Aperture Laboratories the Sentry Gun will disintegrate (and produce an unusual sound) if it passes through a Material Emancipation Grid.
Sentry Guns will also fire at rockets fired from a Rocket Sentry. This is not normally seen in-game, as they normally never encounter each other.