Combine Sentry Gun
The Combine Sentry Gun, also known as Sentry, and referred to by the Overwatch as Sterilizer and Turret by the Resistance, is a fully autonomous tripodal pulse weapon featured in Half-Life 2 and its episodes, commonly used by the Combine to guard chokepoints and vital areas. It is also available in the battle mode of Half-Life 2: Survivor as a placeable item.
The Sentry Gun is one of the many human technologies adapted by the Combine for their own use after the Seven Hour War. Guns are often deployed by the Combine as static defenses, used to guard areas that are infrequently patrolled, or as a means to supplement a larger defense network already occupied by Overwatch forces. 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 Force Field-equipped lockers.
Despite the similarities with the Sentry Gun, the Combine Sentry Gun has a number of key differences from its predecessor. Mainly, its targeting systems is more sophisticated. It does not rely on laser tripwires, instead incorporating a visual sensor into its main body, allowing to 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 Overwatch forces, humans and aliens. Another key difference is that the Combine Sentry Gun has a narrower field of view and range of motion, whereas the Sentry Gun is able to rotate and fire in nearly any direction. The Aperture Science variant has an even more limited field of view.
An individual gun's programming can be accessed and modified remotely through a separate computer terminal when necessary. In Half-Life 2’s 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. In Episode Two’s second chapter, This Vortal Coil, Rebels have a supply of reprogrammed Sentries used to repel Antlions.
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.
On the body the alphanumerics "V952" can be seen. It is also featured on other Combine devices.
In Half-Life 2, Sentry Guns are first seen in Nova Prospekt during the chapter of the first name, where the Combine forces make frequent use of them to slow Gordon Freeman's progress and impede the Antlions he takes with him. Later in the game, in the chapter Entanglement set in the Depot, Alyx Vance reprograms the Turrets so Gordon can use them to protect them while the Combine teleport charges again.
During Episode One, Sentry Guns are featured in the Hospital, protected by a Force Field and Hoppers, with a metal cabinet holding them in place, making them considerably more dangerous and difficult to defeat.
In the Victory Mine during Episode Two’s second chapter, This Vortal Coil, two Resistance members, Griggs and Sheckley, reprogram two Sentries to ignore them and attack Antlions instead. 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 while the Vortigaunts heal a wounded Alyx; however, being damaged and crudely repaired with duct tape and baling wire, they short out and explode in the middle of the attack.
In Half-Life 2: Survivor's battle mode, Sentry Gun is available as a usable item that allows players to place their own Sentry Guns. It is an unlockable item for the Sniper, and is obtained by reaching B-Class. Battle mode's Sentry Guns are team colored for easy identification, friendly Sentries will also have an arrow above them. Engineers can reprogram enemy Sentry Guns by picking them up with the Gravity Gun.
- Sentry Guns cannot be physically destroyed, and must instead be knocked over, which disables them. It can be easily achieved, as it features an unstable tripod base defect that was only noticed after manufacturing. 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. 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.
- Survivor's Sentry Guns also explode in short amount of time after being disabled, so it is advised to stay away from them.
- 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.
- As said above, reprogrammed Sentries come very handy in tough situations. They must be placed at strategic points, where they stand a higher chance of defeating its opponent before it can reach it. When knocked over by Overwatch forces, Sentries must be repositioned as fast as possible. Unlike hostile Sentry Guns, when a reprogrammed Sentry is picked up, it automatically heads upright, which is very convenient during a battle.
- During the chapter Entanglement, it is possible to carry reprogrammed Sentry Guns forward to several areas. First, in cell block D8, where a difficult turret standoff takes place, as many Turrets as possible can be brought from previous areas, which will make the battle much easier. If functional Turret remain after the battle, they can be carried again to the teleportation chamber that follows. One should be careful while doing this, however, since the activated Turrets brought in will shoot at and kill Eli and Mossman. Indeed, reprogrammed Sentry Guns only recognize Gordon and Alyx as friendly, and will kill other friendly NPCs.
- It is not possible to get more reprogrammed Sentries via the console, as they are scripted. Any Sentry Guns spawned that way will be hostile towards the player and friendly NPCs.
 Related Achievements
|Warden Freeman (10G)|
|Survive the second turret standoff in Nova Prospekt.|
|Half-Life 2: Episode Two|
|Into the Breach (5G)|
|Help Griggs and Sheckley hold off the Antlion invasion inside the mine shaft.|
 Behind the scenes
As seen in the model files of the playable Half-Life 2 leak, an unused Sentry Gun model shows it had a some point a different shape. Already a tripod, it was thicker and shorter. The model used in gameplay of the leak is close to the retail model, with some differences.
Ditto. This locker contains 3 reprogrammed Sentry Guns and 3 Supply Crates.
 Half-Life 2: Episode One
 Half-Life 2: Episode Two
Reprogrammed Sentry Gun model, as seen in the Victory Mine.
Sheckley standing next to a reprogrammed Sentry Gun.
 Half-Life 2: Survivor
 List of appearances
- Half-Life 2 at E3 2003: Bugbait (First appearance) (Non-canonical appearance)
- Half-Life 2
- Half-Life 2: Raising the Bar
- Half-Life 2: Episode One
- Half-Life 2: Survivor (Non-canonical appearance)
- Half-Life 2: Episode Two