Z-Sec

The Z-Sec is a closer recreation of the classic zombie, as they carry firearms; Doom 3's zombies do not. Unlike the classic variations, all Z-secs wear black uniforms and armor, possibly in a reference to the classic shotgun guy. They are security guards possessed and mutated by evil spirits; early in the game, an in-game monitor shows a security guard being possessed by a ghostly skull, which causes his skin to turn white and his eyeballs to explode. Several of the Z-secs have their eyes hidden when they're wearing a helmet, but are still able to "see" the player, possibly due to the hellish spirits controlling them.

There are four types of Z-secs, each with its own distinct characteristics and firearms. These are the Pistol Guard, the Shotgun Guy, The Machinegunner and the Riot Shield Trooper.

Combat Characteristics
General:

The Z-secs are notable for their artificial intelligence (AI), which makes them fight like soldiers. They duck behind objects, popping out of hiding to take shots at the player and occasionally rush the player's position. They can also strafe like the player, but only take a single step when they do. Like most of Doom 3's monsters, headshots do more damage than regular body shots. They generally have better aim than their Classic Doom counterparts and are a lot more aggressive, firing more frequently. They have two "alert" sounds; one is a radio-distorted voice, and the other, less pleasant sound effect is gunfire.

Bald guy with no helmet and glowing white eyes, armed with a pistol. Is capable of dodging by rolling sideways, always retaliating after a successful dodge. They occasionally fire multiple shots in rapid succession and are generally decent shots, but of all three Z-sec types, these guys are the ones that are the most likely to miss. Takes around four pistol bullets to the body to kill. Drops a pistol when killed.
 * Pistol Guard:

Similar to the classic Doom shotgun guy, but he wears a helmet and a broken visor, and his eyes are missing. He is slightly tougher than the Pistol Z-sec. Unlike the player's shotgun, the Z-sec shotgunner's weapon has narrower spread, but thankfully, is considerably less powerful than the player's version. Can sometimes rapid-fire at a faster rate than the player. Drops a shotgun when killed.
 * Shotgun Guy:

Wears a closed helmet with an intact, but bloodspattered visor (obscuring his eyes) and a breather, ultimately covering his entire face. Aside from the usual combat behavior of ducking behind objects, these zombies sometimes rush you while firing their machinegun. Occasionally, they aggressively spray the player's position with bullets, effectively keeping you pinned down. Arguably the toughest of the three Z-secs. Drops a machinegun when killed.
 * Machinegunner:

In addition to a pistol, he carries a Riot shield and wears a helmet, protecting the Trooper from attacks. The Riot Shield effectively protects this Z-sec from anything that hits the shield, and being fully armored himself, is fairly tough (similar to the Machinegunner) when hit where his shield does not protect him. AI is the standard Z-sec combat tactics.
 * Riot Shield Trooper:

Tactical Analysis
General: Both comparatively tougher than the classic zombie troops and equipped with better AI, the Doom 3 Z-Secs should never be underestimated, and using cover is extremely important when fighting these zombies. The Machinegun is generally a good basic weapon against these pests and a good tactic is usually to wear them down with concentrated pistol or machinegun fire and then switch to the shotgun in order to finish them off when they attempt to rush the player. It is usually a bad idea to use the shotgun right away as the shotgun has short range and the Z-secs are tough enough to potentially survive a close-range hit, leaving players vulnerable after the initial shot. A headshot with the shotgun will kill a Z-sec instantly, however.

More powerful weapons like the chaingun and the plasma gun are very effective against Z-secs but should perhaps be reserved for tougher monsters or larger crowds of Z-secs.

The Pistol works well against this guy, but the machinegun gets the job done quicker. It is often a good idea to keep a good distance to this guy as he is more likely to miss his mark, when the player is far away.
 * Pistol Man:

This fellow needs to go down as quickly as possible, because he is capable of dealing a great deal of damage if left unchecked. Using the machinegun to soften him up at range and then finishing him off when he gets in close is the way to go, but be careful to only pop out of hiding after he has fired a shot, but more powerful weapons should be more readily considered.
 * Shotgun Guy:

Sprays more lead than the other types, but his bullets aren't too powerful. Patience is a good thing to have against this guy as he tends to pin down the player with prolonged sprays, but it works to wait for him to pop out of hiding, shoot and quickly duck down behind cover before he opens up. It is a bad idea to fire prolonged bursts at him, because he is tough enough to recover swiftly from hits, often enabling him to retaliate swiftly.
 * Machinegunner:

This guy requires accurate aim and cool nerves as his shield is impenetrable. Keeping one's distance is strongly advised if weaker weapons like the machinegun are used. The best way to dispatch of this nasty trooper is to use the plasma gun against him.
 * Riot Shield Trooper: