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, it will just waste time 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, the player should be more careful to take ammo if far away from it when needed, and 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 as much as you can the magazine topped off. 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.

Ammunition Upgrades
Left 4 Dead 2 will likely feature two ammunition upgrades (which are technically referred to as ammunition modifiers).

[[Image:Hud explosive white.gif|25px]] Explosive Ammunition
Explosive Ammunition is a special type of ammunition found in Left 4 Dead 2. 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.

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 clip will explode on hitting any surface, including Infected. Shotguns fire out a slug instead of pellets for every shell of explosive ammunition fired. Exploding ammunition for the M79 may be thermobaric grenades, which are more explosive. A video of explosive ammunition in action.

[[Image:Hud incendiary white.gif|25px]] Incendiary Ammunition
Incendiary Ammunition is a special type of ammo found in Left 4 Dead 2. Incendiary ammunition sets all Common and Special Infected on fire. When the Tank is shot with incendiary ammunition it is only ignited for 5 seconds. Hazmat UCI infected are immune to all sources of fire, and will not be ignited by incendiary ammunition. The bullets still do damage however.

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 clip will ignite any Infected hit. Picking up incendiary ammo for a grenade launcher will make the ammunition incendiary, but as the weapon has only 1 grenade in it's clip and is already powerful enough to kill all CI in one hit in a wide area, the change is mostly negligible. 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.

Ammo Packs
Ammo Packs may appear as med kit replacement in Left 4 Dead 2. It seems they are deployable for use by other team mates, and use the health pack's slot, so you must choose whether to carry a health pack, Defibrillator, or ammo pack.

With the traditionally low stock of health packs found throughout the game, letting players who prefer the full-auto style to pick up an ammo pack when there are no health packs available increases their usability. As seen from the Left 4 Dead 2 achievement list, there is an achievement that requires you to deploy an ammo upgrade then let your team use it.

The ability to save these powerups for a convenient time such as Tank attacks or Witch sightings, seems like a tactical improvement, or that you can use it, and it functions as an ammo pile. It is also possible that these ammo packs could somehow be used to refill the new Heavy Machine Gun, as it has limited ammunition.

L4D2
Armory of One

Deploy an ammo upgrade and have your team use it.

- ???

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.