Pipe Bomb

"Well, this looks dangerous."

- Zoey "Chase this, you zombie bastards!"

- Nick

The Pipe bomb is a powerful homemade explosive that doubles as a diversionary device. The design is based off a stereotypical pipe bomb, but with the gutted remains of a smoke detector taped to it. When it is thrown, it will emit a loud beeping sound and flashing red light for 5-7 seconds, drawing towards it any Common Infected not already in melee range of a Survivor. When the fuse is depleted, it will detonate and kill all nearby Infected. The pipe bomb may also damage Survivors caught in the explosion radius. Pipe bombs can be an extremely useful weapon in many situations. You can use it to defend victims of the Boomer's bile, since the Common Infected summoned by the bile will chase the pipe bomb in preference. It can be used for clearing out an area of Common Infected, for conserving ammunition, and for crippling a horde, particularly when being attacked from multiple directions or during a finale.

Tactics

 * If you come across an open area with many scattered infected, throwing a pipe bomb will draw all of them to it and kill them all, clearing the entire area quickly.
 * 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 your teammates know when you are going to throw a pipe bomb.
 * During a finale or Panic Event, throw the pipe bomb when most of the horde are on their feet. If a Common Infected is climbing when a pipe bomb is thrown, then they will finish climbing before they run towards the pipe bomb and therefore they may not reach it in time.
 * The Common Infected who are not killed by the explosion are stunned, and therefore very vulnerable to 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 buy you sometime to run away and will stun her.
 * In Versus Mode, the distinct beep of the pipe bomb will make them virtually useless against Infected players, as they can 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 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.
 * If timed perfectly, a pipe bomb will stun an oncoming Tank (much like when you melee 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 isn't a wise choice of weaponry to use for distracting Infected during moments where the Survivors have to move. However, it's still best at 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 the corner and you can't shoot him, you can throw a Pipe Bomb to force him to either get away or be killed or stunned.

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.