Combine Sentry Gun

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

- Alyx Vance

Combine Sentry Guns, referred to by Overwatch as sterilizers and as turrets by Rebels, are fully autonomous tripodal pulse weapons 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, after a brief delay, will 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.

On the body the alphanumerics "V952" can be seen, also featured on other Combine devices.

Half-Life 2
Sentry guns are first seen in the chapter Nova Prospekt, where the Combine make frequent use of them to slow Gordon's progress and impede the Antlions he takes with him.

Sentry Guns were also deployed extensively in City 17 after The Uprising.

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, making them considerably more dangerous.

Half-Life 2: Episode Two
In Episode Two, two Resistance members, Griggs and Sheckley, reprogram two sentries to ignore them and attack antlions. 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 guns later prove invaluable in fending off an extended Antlion attack; however, being damaged and crudely repaired with duct tape and baling wire, they short out and explode in the middle of the attack.

Tactics
Sentry guns can be taken down by many weapons, however the ease of this, and the weapon best suited to knocking them over, varies depending on the situation. When stood behind them, it is recommendable to use either the Crowbar or Gravity Gun to knock them over, as this uses no ammunition. When at distances, and when they are stood in front of walls, it is easiest to throw grenades at them, which can often take out more than one at once.

Sentry Guns are powerful enemies but are limited by the simple fact that they can only fire at targets in their sight line and cannot move. When encountered, it is important to consider tactics; the Sentry Gun is immobile, and from behind can be taken down more easily.

In Nova Prospekt, Gordon has control over Antlions, which can be used as cannon fodder against the Sentry Guns and to distract them while the player runs or sneaks 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 Entanglement, it is possible to carry the sentry guns forwards to the Teleportation Chamber. As a result, by the end when trying to defend the teleportation chamber, Alyx and Freeman can have up to eight Sentry guns to defend their positions. One should be careful while doing this, however, since the activated sentry turrets you bring in will shoot at and kill Eli and Mossman.

Trivia

 * Reprogrammed Sentry Guns only recognize Gordon and Alyx as friendly, and will kill other friendly NPCs.
 * If a deactivated Sentry Gun is placed upright, it will re-activate.
 * A glitch occurs when disabling the AI in-game, where doing so disables the sentry's AI permanently. Sentries will not react, even if "AI_Disable" is entered again to resume AI.
 * Despite Alyx saying that she can reprogram the Sentries the first time they are used, the second time they are used, no reprogramming is necessary for use by Gordon.
 * Except for the fact that they can be reactivated by standing them upright, Sentry Guns are functionally identical to the Aperture Science Sentry Turret. However, Sentry Guns lack the friendly voice of the Aperture Science model, and use pulse ammunition instead of conventional bullets: However, if spwaned in Portal, the firing sounds/effect are made for the mentoned turret, and will be bullet effects and sounds.
 * The beta design with temporary textures (see gallery below) for the Sentry Gun was re-used in the Source game "Alien Swarm" as an actual weapon called the IAF Sentry Gun.
 * A spawned Sentry Guns via console will hostile towards the player,Reprogrammed Sentry Guns cannot be spawned by console,because all Reprogrammed Sentry Guns are scripted.

List of appearances

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