Aperture Sentry Turret

Sentry Turrets are tripod robots that appear in every game in the Portal series. They are seen as incapable of independent movement, but can open each side of their chassis (their 'arms') horizontally in order to reveal two gatling guns. The central part of the chassis holds their red eye, which emits a targeting laser. Almost all of the space within the Turret is devoted to storing the ridiculously stacked ammo, explaining the nearly infinite supply of ammo when firing.

When test subjects are out of sight from the Turret's presence, they will maintain a standby state and will remain motionless with their guns retracted. Moving into their angle of sight however, will cause Turrets to deploy their guns shortly after greeting Chell or Atlas or P-body before opening fire. They will continue to fire until the target is dead or moves out of sight. If the latter occurs, Turrets will briefly search for the test subject (as indicated by the movement of the targeting laser), after which they will revert to a standby state.