Pipe Bomb

"FIRE IN THE HOLE!"

- Zoey "Chase this, you zombie bastards!"

- Nick

The Pipe bomb is an IED that can be used offensively, defensively and for diversionary purposes.

The visual design is stereotypical but with the addition of a bright red LED and loudly beeping smoke detector to attract Common Infected when armed and thrown. Pipe Bombs explode five to seven seconds after being deployed. Unlike a real life Pipe Bomb the in-game version explodes smokelessly.

Its blast radius is relatively small, being confined to little over three to four metres on the normal difficulty setting. However since a well deployed Pipe Bomb is usually tightly surrounded with enemies, as many as 20 or more can be killed at once. Since a Pipe Bomb blast can injure a Survivor, care is needed to ensure that team mates are at a safe distance when one goes off.

Pipe Bombs can be an extremely useful weapon in many situations. The Pipe Bomb can be used offensively to clear out an area, defensively against a horde or to draw off enemies from an incapped team mate, and for diversionary purposes such as when disengaging at the end of a finale.

Tactics

 * This weapon is perfect for quickly clearing out an open area filled with loosely gathered Common Infected. A side benefit is that this mission can be achieved without expending too much precious ammo.
 * Apart from the Bile Jar, the Pipe Bomb is the ideal diversionary device to deploy against the horde during the final moments of a finale when sprinting for a rescue vehicle.
 * Deploy a Pipe Bomb to draw enemies away from a team mate who is either incapped or slimed in Boomer bile.
 * Do not waste Pipe Bombs but keep them in reserve for dealing with large hordes. Remember: this weapon's lethality is directly proportional to the number of enemies it can attract.
 * Do not throw multiple Pipe Bombs. Common Infected will run towards the first one thrown and ignore the others.
 * Let your teammates know when you are going to throw a Pipe Bomb. This will prevent them throwing ones themselves and allow them a moment to scamper to safe distances.
 * Do not throw a Pipe Bomb when a large number of enemies are climbing a fence, barrier or wall. They are programmed to complete their climb before turning and running towards the bomb, by which time the bomb may have exploded harmlessly. Enemies only respond if they are standing on the ground.
 * The Common Infected who are not killed outright by the explosion are stunned and thus vulnerable to a follow up attack.
 * The kill radius may vary with the difficulty level.
 * Special Infected and Uncommon Worker Infected are not attracted to Pipe Bombs (the latter ostensibily because they are wearing ear protection and cannot hear the weapon's beeping).
 * Witches and Tanks can be stumbled or stunned by a Pipe Bomb explosion in their close proximity.
 * Witches will be startled if one goes off just inside her personal space.
 * Pipe Bombs are virtually useless In Versus Mode since human opponents will ignore them and use their sounds and lights to evade them.
 * If Infected are near you, or come too close to you while running towards the pipe bomb, they will forget about it and start attacking you. Use your melee attack to push them away which will cause them to disengage, and chase after the pipe bomb.
 * Pipe Bombs are less effective In Left 4 Dead 2 since passive enemies appear to respond more slowly to their stimuli.
 * In Left 4 Dead 2 when a pipe bomb is thrown down a building or bridge the infected will still follow the pipe bomb and plunge to their deaths. This can be used during the start of Dead Center or the finale of The Parish. In Dead Center, you can get a potential double kill with it; by throwing the bomb from the lower floor and through the sky window of the lobby, you can cause the infected on the current floor and those in the lobby to be killed by it, the ones on the current floor will follow the bomb, while it will destroy the ones in the lobby.
 * A pipe bomb combined with Grenade Launcher fire will make a bigger explosion, and may prove useful to killing any remaining stunned Common Infected left out after the detonation.
 * When a Special Infected is around a corner or behind cover and cannot be shot, a Pipe Bomb can be tossed or bounced around the angle to draw him out, injure or stun him. (Note that unlike other FPS games with pre-spawned enemies, Left 4 Dead series players cannot throw Pipe Bombs around corners or into rooms on a speculative basis since The Director spawns enemies randomly and opportunistically.)
 * Throwing a pipebomb on a ledge or a cliff will cause the nearby zombies to jump for it and die. This can be useful in the Hotel area of the Dead Center campaign.
 * If you're about to accidentally throw your only grenade just switch to your primary weapon and you will not throw it.

Behind the Scenes
According to the game's Developer Commentary, the Pipe bomb did not have the flashing light when it was originally conceived; instead, it served as a conventional grenade. There were some issues with this however: rushing Infected would ignore the grenade, and often run past it, making the Pipe bomb useless unless perfectly timed. Another, worse, problem was when the Infected not only ignored the grenade, but kicked it back toward the thrower (not in a literal sense, but it would bounce off of them). Valve fixed the problem by making the pipe bomb attract Common Infected, and added the oxygen tank to perform the role of the earlier pipe bomb.

The Pipe bomb is based off of Counter-Strike's HE Grenade. Its "price" in the pipe bomb's weapon file is the same as the HE Grenade's and the Pipe bomb's sounds are in a folder called "hegrenade".

It is likely that the Pipe bomb was a regular grenade at one time. Hidden in the game's files is a very early grenade icon for the HUD; this grenade looks exactly like the Counter-Strike: Source HE Grenade. This, combined with the earlier info about the Pipe bomb being based off of the HE grenade, means that the Pipe bomb was likely a regular grenade at one time.

One of the proposed uses of the pipe bomb was for it to be an anti-Tank weapon. This ability is discussed in the Developer Commentary and can be seen in this video dated August 27th, 2008. While the video doesn't show the pipe bomb killing the Tank, it portrays it as useful against one.