Combine Sentry Gun


 * "I can reprogram these turrets to attack the enemy." - Alyx Vance
 * "Uh-oh, turrets!" - Barney Calhoun

This Sentry Turret is a fully autonomous weapon featured in Half-Life 2 and its episodes. They are used by the Combine to guard chokepoints and vital areas, and share many aspects of their design with the older model seen in Half-Life, including: a tripod base, a tall, narrow body, and a deadliness with a projectile launcher which often surpasses that of organic allies.

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 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 in small forcefield-equipped lockers.



Combine Turrets, 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 Turret 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 it's target, the turret 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 turret 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 turret 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.

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 turrets, allowing Gordon Freeman to set them up to protect himself against protracted Overwatch assaults.

The turret 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. Turrets 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 turret's targeting systems will malfunction and the turret will shoot wildly and sound its alarm for a few seconds before shutting down.

Half-Life 2
The Sentry Turret 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. Turrets 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.

Episodes One and Two
During Episode One, Sentry Turrets are seen infrequently. Two ceiling mounted turrets are noted as having ran out of ammunition after firing at overwhelming numbers of Zombies and Antlions but still are firing, unaware of their lack of ammo, evidenced by clicking trigger-like sounds when one points at organic things, more evidence that the Combine are losing control of City 17. More are seen in the hospital section, protected by a forcefield and a metal cabinet holding them in place, suggesting the Combine had at least learned a valuable lesson from the debacle at Nova Prospekt.

In Episode Two, two Resistance members, Griggs and Sheckley, reprogrammed two old sentry turrets to fight for them. Bored during their time at the underground outpost, and with nothing else to do, they decorated the turrets with graffiti and nose art. These turrets 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.

Variants
Ceiling Turret

The Ceiling Turret is basically what its name implies, a turret attached to the ceiling of a room. These types of turrets are indestructible and have a wider firing arc. Ceiling Turrets are static, and cannot be moved. They are used to guard important Combine structures (such as the Overwatch Nexus). They are sometimes only activated by laser tripmines. In Episode One, several Ceiling Turrets are seen trying to fire without any ammunition. This proves that turrets don't have an infinite amount of ammunition (although this could be due to a malfunction).

Floor Turret

Floor Turrets (also can be called Ground Turrets) are only encountered in the Overwatch Nexus. They are built into the floor, and have scanners which are used to detect intruders. When a Floor Turret detects an intruder, the Floor Turret comes out of the floor and starts firing on the intruder(s). To destroy a Floor Turret, one must throw or roll a grenade into the turret's hole in the floor, which makes the turret withdraw back into the floor.

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

Turrets are powerful enemies but are limited by the simple fact they cannot move. When you encounter one, step back and consider your tactics, as you have time to think which you are not afforded by mobile enemies. Are there ways around them? Do you have explosives? Are there physics objects you can use to knock them over from a distance or as a shield against gunfire allowing you to get closer?

Where available, using grenades to knock them over from a distance is an effective tactic.

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

During the level Nova Prospekt, you can pick up any reprogrammed turret with the gravity gun and carry it around with you. 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

 * The Combine Overwatch refer to turrets as "sterilizers".
 * Turrets should not be removed from their usual arena. A glitch causes turrets to only recognise Alyx and Gordon as allies killing any friendly AI.
 * Turrets come in different names: Floor Turret and Sentry Gun.
 * There is a glitch, that when you pick up a turret with a Dark Energy Gravity Gun (Can only be acquired from using cheats to get the Dark Energy Gravity Gun), that causes the turret to 'duplicate' (duplicate turrets don't work and have no collision), as well as not being able to pick up the turret. The same glitch with the Dark Energy Gravity Gun also occurs with rollermines