HECU Sentry Gun

This Sentry Turret is a fully autonomous weapon featured in Half-Life and it's expansions. They are commonly used by the HECU 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 turret 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.

Half-Life
Gordon Freeman encounters a Sentry Turret 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 turret relies upon an independent laser sensor to detect targets. Once a target has tripped the beam, the turret deploys it's 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 turrets 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.

Variants
Besides the standard, portable, tripod-mounted floor turrets, there are some other varieties. Ceiling mounted turrets of various size are occasionally seen in the Black Mesa Facility. These turrets possess no target identification system, and once activated, they simply fire on anything within range. These turrets can either be destroyed with explosives or gunfire, or disabled by switching them off, depending on access.

Tactics
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?

Since Half-Life's Goldsource engine lacks a robust physics system, turrets simply fall over once they've taken a set amount of damage. Half-Life turrets 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; if you can avoid tripping them you can bypass fighting the turrets entirely.