Combine Sentry Gun

"I can reprogram these turrets to attack the enemy. You set them up to defend the control room."

- Alyx Vance

The Combine Sentry Gun, referred to by the Overwatch as Sterilizer and simply as turret by the Rebels, is a fully autonomous pulse weapon used by the Combine to guard choke points and vital areas.

Overview


Turrets are one of many human technologies adapted by the Combine for their own use after the Seven Hour War. The guns are often deployed as static defenses, used to guard areas that are infrequently patrolled, or as a means to supplement a larger defense network. The guns are fitted with small handles and are light enough to pick up and place by hand. When not in use, they are generally stored in small forcefield-equipped lockers.

Combine Sentry Guns, however, have a number of key differences from the model used by the HECU in Half-Life. Mainly, their targeting systems are more sophisticated. The Combine Sentry Gun does not rely on laser tripwires, instead incorporating a visual sensor into its main body, with which it can immediately detect intruders in its sight line. Once activated, it will sound an alarm and open fire. Upon losing sight of its target, the gun will scan the area ahead by moving its upper body back and forth, before returning to its inactive state if the target does not reappear after several seconds. This variety of gun also incorporates a limited A.I. which is somehow able to distinguish between Combine forces, humans and Xen aliens. Another key difference is that the Combine Sentry Gun has a narrower field of view and range of motion, whereas the HECU Sentry Gun is able to rotate and fire in nearly any direction.

An individual gun's programming can be accessed and modified remotely through a separate computer terminal when necessary. In Chapter 9a, Entanglement, Alyx Vance reprograms a number of Sentry Guns, allowing Gordon Freeman to set them up to protect himself against protracted Overwatch assaults.

The Sentry Gun is loaded with the Combine's ubiquitous pulse ammunition, of which it has an essentially limitless supply, although in Episode One, some ceiling-mounted variants with no ammunition remaining are encountered in a zombie infested underground. Sentry Guns cannot be destroyed, and must instead be knocked over, whether by explosives, gunfire, the Gravity Gun, or by getting close to them and knocking them over manually. Once knocked over, the Sentry Gun's targeting systems will malfunction and the turret will shoot wildly and sound its alarm for a few seconds before shutting down.

Its unstable tripod base defect was only noticed after manufacturing.

Half-Life 2
The Sentry Gun is first seen in Half-Life 2's ninth chapter, Nova Prospekt, where the Combine made frequent use of them during Gordon Freeman's invasion of the facility. Sentry Guns were also deployed extensively in City 17 after the uprising, including floor and ceiling mounted variants, which were used to defend important facilities such as the Overwatch Nexus.

Half-Life 2: Episode One
During Episode One, Sentry Guns are seen in the hospital, protected by a force field, with a metal cabinet holding them in place.

Half-Life 2: Episode Two
In Episode Two, two Resistance members, Griggs and Sheckley, reprogrammed two old Sentry Guns to fight for them. Bored during their time at the underground outpost, and with nothing else to do, they decorated the Sentry Guns with graffiti and nose art. These Sentry Guns later proved invaluable in fending off an extended Antlion attack; however, being both old and faulty they shorted out and exploded in the middle of the attack.

Tactics
The Sentry Gun's single greatest enemy is the grenade. Punt heavy objects at Sentry Guns to knock them down, or, if possible, approach from behind or the side and crowbar them over.

Sentry Guns are powerful enemies but are limited by the simple fact they shoot behind them and cannot move. When Gordon encounters one, step back and consider tactics; the Sentry Gun is immobile. Look for a way around, use explosives, throw a physics object at it, or use a large object as a shield.

In Nova Prospekt, Gordon will have control over Antlions, which can be used as cannon fodder against the Sentry Guns. They can be used to distract the Sentry Guns while the player run with them or sneak around. If there are enough Antlions, and one or more survives the Sentry Gun's constant shooting, they will attack the Sentry Gun like they would any other enemy, and knock it over, thus shutting it down.

During the chapter Nova Prospekt, Gordon can pick up any reprogrammed Sentry Gun with the Gravity Gun and carry it around. It will continue to to fire on enemies, saving ammunition and killing faster than most of the weapons the player has at his disposal.

Trivia

 * Sentry Guns should not be removed from the arena they appear in. A glitch causes Sentry Guns to only recognize Alyx and Gordon as allies and will kill any other friendly AI.
 * Sentry Guns can be used to protect a barricade.
 * If a deactivated Sentry Gun is placed upright, it will re-activate, this can be seen in Nova Prospekt. If Gordon picks a friendly turret up with his Gravity Gun, it will automatically hold it upright, since sentries are often the only thing keeping Gordon alive and can be notoriously hard to erect by conventional means.
 * However, in rare cases, if one tosses an inactive Sentry with the Gravity Gun, and the Sentry lands upright, it will activate, but will not be reprogrammed (except in Nova Prospekt).
 * Sentries will not work if you put ai_disable (meaning every ai will stop) and retype it again in the console
 * They share many aspects of their design with the original Half-Life Sentry Gun - a tripodal base, a tall, narrow body, and a proficiency that often surpasses that of organic NPCs.

List of appearances

 * Half-Life 2
 * Half-Life 2: Raising the Bar
 * Half-Life 2: Episode One
 * Half-Life 2: Episode Two