Pipe Bomb

"FIRE IN THE HOLE!"

- Zoey "Chase this, you zombie bastards!"

- Nick

The Pipe bomb is a powerful homemade explosive device comprising a sealed length of water pipe packed with black powder and a percussion cap detonator triggered by a timing device. In a clever act of creativity on the part of its orginal designer, each Pipe Bomb is fitted with the remains of a 9 volt dry cell battery powered domestic smoke alarm and flashing red light causing these devices to emit a loud beeping sound for five to seven seconds that attracts Common Infected in its audible range to armed Pipe Bombs, thereby increasing their lethality.

In-game the Pipe Bomb serves two purposes: as an offensive grenade and as a defensive diversionary device. When one explodes, as many as 20 or more Common Infected can be slain at once making them extremely useful in many situations facing the Survivors. Common applications include: cleaning out an infested area, diverting a horde (or multiple hordes), providing an invaluable diversion during a finale's bug-out phase and defending Boomer bile victims from a mass attack.

As with any explosive device, care must be taken to avoid throwing a Pipe Bomb and inadvertently causing health damage to other team mates - either directly or in the collateral sense by causing Gas Cans or Explosive Barrels to explode in their vacinity.

Special Infected and Worker Uncommon Infected do not respond to the Pipe Bomb's sound and light but Witches can be startled if one goes off too close to her. Relatively slow starting Special Infected Tanks and Witches can be stunned by a nearby Pipe Bomb explosion thus allowing teams time to either pull back to a better fighting position or to deploy and cone them with gunfire.

Tactics

 * Deploy a Pipe Bomb to clean out an open area heavily populated with Common Infected (e.g. many parts of the Crash Course are ideal for this offensive ploy.) A Pipe Bomb will attract enemies into a scrum over the bomb and kill, wound and cluster them to the team's advantage.
 * Hold on to pipe bombs until attacked by a large horde. The effectiveness of the pipe bomb is proportional to the number of Infected nearby.
 * Throwing multiple pipe bombs at once is inefficient, as Common Infected tend to run towards the first one thrown. Let teammates know when you are going to throw a pipe bomb so that they do not launch bombs themselves and can duck back out of its blast radius.
 * During a finale or Panic Event, throw the pipe bomb when most of the horde are on their feet. Any Common Infected who is climbing when a pipe bomb is thrown will finish climbing before running towards the pipe bomb - the risk is that the bomb will explode before they enter its blast radius.
 * The Common Infected who are not killed by the explosion are stunned or wounded and are therefore very vulnerable to follow-up attack.
 * The kill radius may vary with the difficulty level.
 * Special Infected, in campaign or single-player, will not be attracted by the pipe bomb. However Witches can be startled if the explosion occurs too close to her personal space. Molotovs or gas cans are preferable when faced with Special Infected. Throwing a Pipe Bomb at the Witch can nevertheless buy time to run away since it will momentarily stun her.
 * In Versus Mode, Pipe Bombs are virtually useless against Infected players since the beeping noise alerts human enemies so that they can then easily avoid them or even run right past them.
 * During a finale, it is always good for at least one player to hold onto a pipe bomb until the rescue vehicle arrives, then to use it. This will distract the horde long enough to get in the rescue vehicle.
 * If Infected are very close to a human player or are obliged to pass by closely when responding to a Pipe Bomb's sound and light, they will forget about the bomb and start attacking the player. Using melee attack in such instances to push them away will cause them to disengage and resume their pursuit of the pipe bomb.
 * If timed perfectly, a pipe bomb will stun an oncoming Tank (much like when meleeing a Common Infected). The Tank will become stunned and knocked back a little, long enough for you to fill him with lead.
 * In Left 4 Dead 2, the pipe bomb is not an optimal choice of weaponry to use for distracting Infected during moments where the Survivors have to move since Infected take a longer time to respond to its sound and light. However, it is still the best way of stopping oncoming hordes, and is better than the bile bomb for stopping rushes from teammates covered in Boomer bile.
 * Pipe bombs especially come in handy when a teammate is incapacitated, if a teammate is unreachable (covered by the horde) this is the pipe bomb's time to shine; throw it and while the horde is away, revive your teammate(s).
 * 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 the ones in the lobby will be destroyed by it.
 * 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.
 * The Pipe Bomb seems to have gotten less useful in L4D2, although rag doll effects do improve it visually, it seems to take longer for infected to target it and thereby gives them less time to run to it.
 * When a Special Infected is around a corner and cannot be shot, a thrown Pipe Bomb will force him to either get away or be killed or stunned.
 * Unlike certain other FPS games (e.g. Far Cry and Half Life) where enemies persistently pre-spawn at specific locations, the ability of players to bounce Pipe Bombs (and other grenades) around corners, toss them blindly into rooms and over ledges on a predictable and pre-emptive basis is not possible in the Left 4 Dead series. Instead, this tactic is only really possible when a visual sighting is confirmed (for example, a Tank's flapping arm is glimpsed around a corner).

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.