Ammo

Ammunition (colloquially known as Ammo) piles are resources found in safe rooms and throughout chapters that contributes to a Survivor's amount of ammunition in their current firearm.

They can be used to restock on ammunition for primary weapons, which include: There are two types of normal ammo containers, while they look different and in visual look they have different bullets, both of them can be used for any weapon. The pistol and the minigun both have unlimited ammo, and as such, it is not necessary to retake ammo.
 * Pump shotgun
 * Submachine gun
 * Assault rifle
 * Hunting rifle
 * Auto shotgun
 * Chrome Shotgun
 * Silenced Submachine Gun
 * Combat Rifle
 * Sniper Rifle
 * Combat Shotgun
 * AK-47

Tactics

 * All safe rooms have ammo piles. Use this to your advantage.
 * Always make sure you have full ammo before leaving a safe room.
 * Fire at enemies while in the safe room, then restock on ammo before you leave.
 * You may want to go back to a safe room for more ammo, but only if you are close.
 * There is no point running back through half the map just for more ammo, as the Director will probably spawn multiple hordes to and from the ammo pile. This will defeat the purpose of getting ammo and might lead to a complete loss. Use your pistols until you reach the next ammo pile.
 * Always take more ammo if possible and reachable, every bullet counts to your success. Just make sure to go with an ally to avoid death.
 * In Crescendo Events, players should try to stock on ammo before it begins. Do not venture towards it alone, though. This could lead to you being ganged up on and incapacitated. Always try to only reach the ammo pile with all available Survivors.
 * In finales, try to reach the ammo pile before the Tank arrives.
 * Ammo piles will never run dry. Clear out the area ahead of the ammo, then restock.
 * Ammo is usually found near Crescendo Events and on longer levels.
 * Dead Survivors drop their weapon with the same amount of ammo they had, use this as an advantage to "restock" on ammo if needed.
 * If you are concerned about your ammo, develop a habit of switching to your pistols often. They're quite useful and can fire very fast with repeated tapping of the fire button, especially for Shotgun/Hunting Rifle users.

Ammo Management

 * The pump shotgun is likely the easiest weapon to manage. Pumping means that you can easily keep track of your shells. Use pistols for medium to long ranges, or if you are a terrible shot. Don't snipe with this gun, it is not going to hit anything beyond close range.
 * The auto shotgun is trickier than the pump shotgun. Use tactics from all of the other weapons. Fire slowly, but effectively, keep an eye on the ammo counter, avoid using all the magazine to avoid the cooldown after reaching 0 bullets, and keep the magazine topped off as much as you can . Save the rapid fire for Tanks, Witches, and hordes. Holding down the fire key will continually fire this weapon but it is slower than rapidly hitting the fire key. Use this to keep yourself from wasting shots. Like the pump shotgun, use pistols for longer ranges.
 * The hunting rifle is the middle ground of all the weapons. The single shot, high power, and high reserve means that ammo is seldom a problem. However, the allure of rapid fire can trick many a player. It only takes one shot from this weapon to kill Common Infected. Reload often, and fire slowly. Use the scope for distance and accuracy. Never fire while moving. Accuracy is greatly reduced with motion. Use pistols for run and gun situations.
 * The submachine gun chews up ammo like crazy. Fire in bursts at medium to long range. 3-5 rounds per burst should be sufficient. Save full auto for large hordes, big threats like a Tank or Witch and close encounters with boss infected. Take periodic glances at your ammo count and plan how to use those remaining rounds.
 * The assault rifle follows the same general rules as the submachine gun, except the reserve is 120 rounds smaller. Increased power means that bursts of 2-4 rounds should work. Check your reserves more often than you would with the submachine gun. Continue to save full auto for those big threats. Try to shoot Common Infected in the head for instant kills.

Ammunition Upgrades
Left 4 Dead 2 features two ammunition upgrades (technically referred to as ammunition modifiers) for all primary weapons. This powerful ammo is rare and usually only found a handful of times in each campaign. Like other weapons and items in Left 4 Dead 2, it can spawn in many different places throughout the environment, depending on the whims of the AI Director. When picked up it occupies the Medkit slot. To use the ammo, the player first deploys it on the ground (which takes several seconds and leaves them vulnerable to attack), after which each player can use it once to get their share of the ammo. Both of these ammo types can be used with the Grenade Launcher, meaning you can pick up that kind of ammo, both of these upgrades increases the size of the blast a great deal, incendiary ammunition also ignites dead bodies.

Explosive Ammunition
Explosive Ammunition instantly kills all Common Infected and knocks down anything close enough to the explosion. The ammo causes massive damage to the infected, destroying a large amount of torso and midsection depending on shot placement. It can also stumble Special Infected and can release a pummeled Survivor from a Charger's grip. If you shoot an Infected too close to yourself you will stumble and take a little bit of damage. On Versus, the impact of explosive ammo on a human-controlled Special Infected will cause the infected to stumble a bit as though they have been melee'd and their ears will also ring.

After picking up this type of ammo, all bullets for both tier 1 and tier 2 weapons for the duration of a magazine will explode on hitting any surface, including Infected. Shotguns fire out a slug instead of pellets for every shell of explosive ammunition fired. A video of explosive ammunition in action.

Oddly the ammo boxes carry 5.56mm, buckshot and 9mm but work on all guns.

Shooting at targets point blank with explosive ammunition will inflict damage on yourself and friendly fire to nearby teammates on higher difficulty settings. Thus when swarmed it should not be used.

Incendiary Ammunition
Incendiary Ammunition sets all Common and Special Infected on fire, with the exception of the Hazmat infected. When the Tank is shot with incendiary ammunition it is only ignited for 5 seconds. Hazmat infected are immune to all sources of fire, and will not be ignited by incendiary ammunition.

After picking up this type of ammo, the player will reload, and all bullets for both Tier 1 and Tier 2 weapons for the duration of a magazine will ignite any Infected hit. Picking up incendiary ammo for a grenade launcher will give it an incendiary grenade that causes the explosion to almost double in size and also ignites the body parts flying away, but since only one grenade will be incendiary, a player has to plan carefully when to use it. A video of incendiary ammunition in action.

Surprisingly, this is not just limited to Left 4 Dead 2 only. If you happen to find a server in the first Left 4 Dead that contains a special point system designed for the server (using a source mod), it is sometimes possible to be able to get incendiary ammunition by using those points to buy it, for a limited amount of time. Most of these servers are hard to cross upon however.

Note that you can set a Tank on fire with incendiary ammo, but it will extinguish after a few seconds (unlike being lit by other sources of fire, e.g. molotov or canister) thus causing not much damage. It is useful to have one player only shoot at the tank in short bursts so that he is relit all the time and cause fire damage while the rest of the team unloads to cause bullet damage.

Behind the Scenes
Originally in Left 4 Dead 2, there was an item called the Ammo Pack. They would be placed in the Medkits slot when picked up and used a reskinned First aid kit model. Much like the current Incendiary and Explosive ammo packs, they could be deployed for use by teammates, with which they could resupply their ammo. This was cut because it went against the idea of players having to pick up new weapons when their current weapon was low on ammo and because play testers would deem ammo packs less usable so long as ammo piles still existed. Its texture sheet can still be found in Left 4 Dead 2 materials files. Typing 'give ' in the console will show that one can type 'give ammo_pack', however, the game will not create one. The deployable idea was kept for the Incendiary and Explosive ammo upgrades.

The two ammo upgrades were originally designed to be static items that the Director placed in the world. However, testing showed that players were unwilling to move away from an area that had ammo upgrades, so they made it so that ammo upgrades could be picked up by players and placed anywhere, much like the previously described ammo pack.

Left 4 Dead 2
Armory of One

Deploy an ammo upgrade and have your team use it.

-Use an ammo pack, and have every survivor take some of the ammunition.

Burning Sensation

Ignite 50 Common Infected with incendiary ammo.

-Simply find or use an incendiary ammo cache and shoot the Common Infected. They are ignited instantly.

-For quick kills, throw a bile bomb and start shooting once they come. This method applies to the pipe bomb as well, however begin by shooting the infected farthest away first (the least likely to be killed by the pipe bomb's blast).

Trivia

 * Oddly, the Survivors in Left 4 Dead 2 will call ammo piles "weapons" or "guns" instead of "ammo".
 * In Left 4 Dead 2, the pile of ammunition is the only ammo model; the tin cans of ammunition found in Left 4 Dead do not naturally exist.
 * The model for a deployed ammo pack is three open boxes of the ground model of the ammo pack. This is odd for two reasons: There are four shares of ammo for each Survivor, yet only three ammo boxes. Each ammo box seems to hold enough rounds for a single clip of an Assault Rifle or Submachine Gun. The other odd thing is that one ammo box somehow becomes three.