HECU Sentry Gun

The HECU Sentry Gun is a fully autonomous weapon featured in Half-Life and its expansions. Sentry Guns are commonly used by the HECU and Black Ops to guard chokepoints and vital areas, and share many aspects of their design with the newer model seen in the sequel, Half-Life 2, including: a tripod base, a tall, narrow body, and a deadliness with a projectile launcher which often surpasses that of organic allies.

Overview
The gun consists of a powerful machine gun built onto a tripod. The gun itself rests on a turntable mechanism of sorts on the top of the tripod, which allows it to rotate and fire in any direction, unlike the equivalent device in Half-Life 2, which has a narrow field of fire.

Gordon Freeman encounters a Sentry Gun for the first time in the chapter "We've Got Hostiles". These were used by the HECU to guard areas after their soldiery had passed through. The gun relies upon an independent laser sensor to detect targets. Once a target has tripped the beam, the turret deploys its weapon with a high pitched whine, and fires upon any target in sight. Once activated, the sentry would remain alert for a time (it makes a steady beeping sound whilst scanning for targets) before sinking into a dormant but alert state. Despite appearing gawky and unstable, these guns are in fact stronger than they appear, and are difficult to knock over. Enough of any type of damage will knock them over, but explosives are the most effective counter.

A variant is encountered later on that auto-arms when it detects motion. The Playstation 2 version put a red light on these to indicate that they were motion-activated rather than sensor-activated.

Tactics
Sentry Guns are powerful enemies, but are severely limited in mobility. When encountering one, stepping back and considering tactics is advise, as unlike other enemies, this one does not move. Looking for a way around or using explosives is a good move.

Since Half-Life's GoldSrc engine lacks a robust physics system, Sentry Guns simply fall over once they've taken a set amount of damage. Sentry Guns can take a considerable amount of damage, so opt for hard-hitting attacks, like a well-placed .357 Magnum shot, a Shotgun double-blast, or explosives, like Grenades, Satchel Charges, or the RPG. Also, keep an eye out for their laser sensors; avoid tripping them, bypassing fighting the guns entirely. However, these tactics may change when playing Half-Life: Source because of the upgraded physics engine.

List of appearances
]]''
 * Half-Life: Day One
 * Half-Life
 * Half-Life: Opposing Force
 * Half-Life: Blue Shift
 * ''[[Half-Life:seven coop