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. Always 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.
 * The Ammo Piles cannot refill ammunition for the Grenade Launcher, so try to preserve as much as the ammo as possible for Boss Infected, Hordes or when Vomited/Exploded on by a Boomer. Also, the Grenade Launcher has a very large radius so it is best used from long range, try to stick to using Pistols or Melée Weapons.

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.

Incendiary Ammunition
"Let's burn some shit UP!"

- Coach

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.