Headcrab Shell

The Headcrab Shell, also known as "headcrab canister" or "headcrab rocket", is one of the more nefarious pieces of technology possessed by the Combine. These shells rain down from the sky and release a payload of deadly headcrabs. Due to its use of living organisms in attacks, headcrab shells are classified as a form of biological weaponry.

Overview
These large projectiles can crash through most barriers, embedding themselves several feet, even into concrete. At this point a hatch at the rear of the shell opens and reveals a metallic pipe-shaped container. There are several hoses that seem to spray the headcrabs with some sort of gas or mist, possibly to enrage them or to speed up zombification. Up to a half-dozen headcrabs climb from the end of this pipe and leap out into the crash site. Usually these are only basic headcrabs although fast headcrabs have been observed exiting a shell. Poison headcrabs are also used in shells, as seen in Lost Coast and Episode One, but only one of them is included per shell.

Once landed the headcrabs go about their usual practice of attempting to kill or enslave any human they encounter. The headcrabs will then simply infest the surrounding area, concealing themselves by burrowing into the nearby soil or taking up residence in pipes, vents, and other darkened hiding places. This infestation renders a significant area surrounding the crash site completely inhospitable to human life. While one shell is not a huge threat by itself, as headcrabs are generally weak, the effects can be horrific when the Combine shell an area with scores of these munitions. This happened to the mining town of Ravenholm prior to the events of Half-Life 2 which was inhabited by a large number of escapees from City 17 and was quickly overwhelmed by the parasites, to the point where there is only one known human survivor, Father Grigori, left in the town.

Throughout Half-Life 2, headcrab shells are fired from an unseen launcher somewhere in the player's vicinity. In Lost Coast, a working launcher is seen in St. Olga's monastery. The headcrab shells are fired with a loud explosion and streak towards their target in a steady arc, leaving temporary vapor trails behind and emitting a noise not unlike a conventional mortar shell.

The launcher itself has a large metal barrel with a plunger inside which fires headcrab shells roughly every 10 seconds. A sign on it indicates, "Warning, Moving parts can crush and cut. Keep hands and other objects clear during operation." It is easily destroyed by dropping metal objects like candlesticks into the chamber. Even though the Lost Coast chapter was cut, it is very likely that this depiction of the launcher is canon.

In "Water Hazard", headcrab shells which don't deploy anything are still shown to be very powerful. One can be shown toppling a massive smokestack, almost crushing Gordon in his airboat.

The Combine uses headcrab shells in much the same way as they use manhacks, only on a wider scale, being an efficient way to eliminate entire centers of Resistance. In addition to being used on their own as an effective weapon, individual headcrab shells are also used in a tactical role during some operations where they are used to weaken smaller targets, create disarray, and force enemies out of entrenched positions.

Despite these horrific implications, this particular Combine technology does have one major weakness. Since the headcrabs are effectively bottlenecked into the opening, they can be defeated easily before they are able to climb out of the shaft by simply standing near it and shooting or beating them to death as they jump out.

Behind the scenes

 * The concept was first intended for the cut Sacktick and as a device attached to a Gonarch, the Big Momma Pod.