Shotgun guy

The Sergeant (sometimes called the Shotgun guy or Shotgun sergeant) is a marine who has been turned into a zombie during the invasion of the UAC bases. He appears as a bald, muscular man wearing bloodstained, slate-gray clothing, and wields a shotgun.

The Doom instruction manual says:
 * Same as the Former Humans, but much meaner, and tougher. These walking shotguns provide you with a few extra holes if you're not careful!

In the instruction manual he is referred to as a Former Human Sergeant.

The sergeant is first encountered on E1M1: Hangar (Ultra-Violence and Nightmare!) or E1M3: Toxin Refinery (I'm too young to die, Hey, not too rough, and Hurt me plenty).

Combat characteristics
The sergeant attacks by firing his shotgun, which is rather less powerful than the player's shotgun: one round produces three pellets instead of seven, and each pellet inflicts slightly less damage on average. When killed, he drops the shotgun, which contains 4 shells (8 on I'm too young to die and Nightmare!).

Tactical analysis
Due to the "spread" of his shotgun pellets, the sergeant is only a marginally greater threat than the trooper at very long range. However, at close range the sergeant can damage the player significantly, and many third-party levels place sergeants directly behind doors and around corners in order to ambush the player.

The sergeant will engage in infighting against all other monsters, including other sergeants. However, the sergeant has only slightly more hit points than the trooper, and tends not to survive monster battles. The sergeant does not have an alternative attack; he cannot punch.

The player's shotgun can kill several sergeants with one shot at close range, and will generally kill individual sergeants at medium and long ranges. All other weapons are effective against sergeants, although the pistol, fists and chainsaw are risky on account of the sergeant's shotgun.

When killed, the sergeant's chest explodes, assuming he is not gibbed. Due to a bug in Doom's engine, the very first sergeant that the player encounters in MAP02, whilst playing Vanilla Doom II, is blind and will not react to the player unless he hears a sound. 

Appearance statistics
The IWADs contain the following numbers of sergeants: