Sentry

Sentry
First Seen: Half-Life 2, Chapter Nine, Nova Prospekt
 * Health total: N/A
 * Weapon damage: Pulse gun (3)
 * Entity:

Sentries (referred to by the Combine as "sterilizers") are automated gun turrets mounted on a tripod. The Combine sentry shares a similar appearance and function with those used by the fictional Hazardous Environment Combat Unit of the United States military in the original Half-Life. However, the Combine version does not use secondary laser "tripwire" detectors, using motion detection (or optical) sensors instead. The Combine sentry also has a narrower field of view and range of motion, whereas the HECU's turret is able to rotate and fire in nearly any direction.

The sentry's gun is loaded with the Combine's ubiquitous pulse ammunition, of which it has an essentially limitless supply, although in Episode One, some with no ammunition remaining may be found in game. Sentry guns use a friendly fire detection system, allowing them to differentiate between Combine, alien and citizen. An individual gun's programming can be accessed and modified remotely through a separate computer terminal when necessary. Note that in Chapter 9a, Entanglement, Alyx reprograms a number of sentry turrets, allowing Gordon to deploy them as defense against oncoming hordes of Overwatch soldiers.

The guns are often deployed as static defenses by the Combine, used to guard areas that are infrequently patrolled, or as a means to supplement a larger defense network. The turrets are fitted with small handles and are light enough to pick up and place by hand. When not in use, they are generally stored locked in small forcefield-equipped lockers. While the sentries appear to be indestructible, they are unable to target properly when knocked over (this is quite easy to do, as the turrets have a high centre of gravity, and narrowly-spaced legs.) When this happens, they automatically set off a high pitched alarm and fire in random directions for a couple of seconds, before shutting down until righted.

Against non-Combine enemies, assuming the sentry is still under Combine control, the gravity gun can be used to pick up the sentry from the rear, allowing direction of the firepower of the sentry gun to be in front of the player, shooting at any available enemies (except Combine units). However, using this tactic in the company of allies will cause them to be fired upon as well.