Ammo

Ammunition piles (colloquially known as Ammo) are found in Safe Rooms and scattered elsewhere throughout the game. They can be used to restock the reserve ammunition of primary weapons, though special tier weapons (the Grenade Launcher and M60) cannot be reloaded. The various Pistols, the Minigun, and the Heavy Machine Gun all have unlimited ammo. There are two visually different ammo piles, a pile and a bucket full (only one in Left 4 Dead 2 though, the pile) but both have the same mechanic.

Tactics

 * Ammo piles will never run dry.
 * All safe rooms (except the starting saferoom of The Bridge in Left 4 Dead 2) have ammo piles. Players should always use this to their advantage by:
 * Ensuring they are carrying a full ammo loadout before leaving a safe room.
 * Firing at enemies while in the Safe Room then restocking on ammo before leaving.
 * Returning to a very close-by safe room for more ammo.
 * It is rarely a good decision to return to a distant safe room for more ammo as the Director will invariably spawn and interpose multiple hordes and Special Infected during the transits back to and subsequently from the safe room: all of which negates the purpose of the journey. Players running low on Primary weapon ammo should instead switch to pistols or melee weapons and push forward to the next safe room.
 * In Crescendo Events, players should try to stock up on ammo before the crescendo begins then restock their ammo as soon as possible after the event.
 * In finales, try to reach the ammo pile before the Tank arrives.
 * Outside of safe rooms, ammo is usually found near the venues for Crescendo Events and at intervals on longer levels.
 * If a player is are concerned about a looming Primary weapon ammo shortage, it is imperative to switch to pistols or a melee weapon in order to tackle low and moderate threat events.
 * Ammo Piles cannot refill ammunition for the Grenade Launcher or M60 so players equipped with these weapons should either preserve as much as the ammo as possible for Special Infected or hordes or (if running short of ammo) drop them for an alternative fully supplied Primary weapon.
 * Be aware: In Left 4 Dead, ammo piles and cans will restock the maximum possible in reserve and not account for partially filled magazines in a player's weapon. A player with a part-filled magazine has to remember to make a pickup, reload the magazine and then make another pickup to secure a full ammunition loadout. In Left 4 Dead 2, ammo restocking under this condition is simplified.  Ammo piles will restock the maximum possible in reserve and temporarily add the amount a player is missing from a part-filled magazine (if present) to that player's reserves;  meaning maximum ammo is obtained via only a single pickup.  Using a Combat Shotgun as an example, it holds 10 shells and 90 in reserve. If a player has 1 shell loaded and 75 in reserve, the ammo pile will give 99 reserve shells thus topping off the shotgun even though it has not yet been reloaded.

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. Do not snipe with this gun if playing in Normal difficulty or higher; it is not likely to hit anything beyond close range. In Left 4 Dead 2, the Pump and Chrome Shotgun have a severely reduced reserve capacity, requiring more careful conservation of ammunition and switching to backup weaponry.
 * 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 zero shells, 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. In Left 4 Dead 2, the Auto Shotgun (renamed Tactical Shotgun), as well as the Combat Shotgun, have reduced reserve capacity, though not as severe as the tier 1 shotguns.
 * 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. If moving while firing, the accuracy of the Hunting Rifle is reduced. When The Sacrifice was released, the accuracy of the Hunting Rifle (previously with extremely reduced accuracy when moving) was balanced to match the Left 4 Dead 2 version. In Left 4 Dead 2, the Hunting Rifle has reduced reserve ammunition, but has had its decreased accuracy while moving reduced to more manageable levels compared to the original Hunting Rifle from Left 4 Dead for balance. The Sniper Rifle functions the same as the Hunting Rifle, though it holds double the amount of rounds in the magazine, and thirty more rounds in reserve, making it slightly more ammunition-efficient, but reloads slower.
 * 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. In Left 4 Dead 2, the Submachine and Silenced Submachine Gun have roughly a 35% increase of reserve ammunition, increasing their effectiveness, as they can be used longer, though they should still be used in bursts.
 * 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 generally get the job done, though if trying to conserve ammo aim for the head. 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. In Left 4 Dead 2, the reserve ammunition remains unchanged, though the size of the magazine for the Assault Rifle (renamed M-16 Assault Rifle), AK-47, and Combat Rifle vary in size, as well as firing rate, power, and accuracy, requiring a slight change of tactics for each gun.
 * The Grenade Launcher is somewhat easy to keep track of in the sense of how many shells you have used and have left over, due to its one shot capacity. Reloading frequently makes most players check their ammunition, which makes keeping track of it easy. Though this weapon is the one with the lowest amount of shells (1 in the chamber and 30 in reserve) when found, so even with its slow rate of fire a Survivor can still burn through their ammunition quickly. When using this weapon try to only fire shots at very large groups of Infected or when a horde is spawned by the Director. The shell will kill a very large number of Infected if used properly and if this weapon is used for only large groups or Special Infected, the user can potentially get through an entire campaign without running out of munitions for it. Be careful though when using the Grenade Launcher as it deals massive amounts of splash (explosive) damage to you and your team. This can be devastating on harder difficulties.
 * The M60 is similar in function to the Assault Rifle and its variants, but all of its reserve bullets (of which there are 150) is placed right into its magazine, disabling the need for reload. As the bullets of the M60 is extremely powerful, able to kill Common Infected in a single shot, care should be used for single shot, or short burst fire for Special Infected, while saving full auto for the Witch, Tank, and hordes. The M60 cannot be supplied with fresh ammunition, so in order to conserve the magazine, making use of secondary weapons such as Pistols or a melee weapon is recommended.

Ammunition Upgrades
Left 4 Dead 2 features two ammunition upgrades for all primary weapons. This powerful ammo spawns randomly, usually only found a handful of times in each campaign. When picked up it occupies the First aid kit 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. This gives them one extra magazine loaded with the special ammunition, except in the case of the M60 where all of its remaining ammo becomes Frag/Incendiary ammo, after which the gun is then discarded.

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

- Coach

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Official Description
Equip your existing primary weapon with these incendiary rounds to ignite any infected upon contact. The next best thing to a Molotov against an angry Tank.

Incendiary Ammunition, also referred to as Fire Bullets by the Survivors, sets all Common and Special Infected on fire, with the exception of the Hazmat Infected. Infected CEDA Workers are immune to all sources of fire, and will not be ignited by incendiary ammunition in any circumstance. 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.

Since incendiary ammunition is a one-hit kill against Common and Uncommon Infected on all difficulties, it is advised to conserve your fire and attempt to only hit each Infected once. Incendiary rounds are less effective when applied to Sniper Rifles and one-hit kill weapons, as it adds no power and the burning, dying Infected can obscure your view.

In the case of the Grenade Launcher, Infected who are not killed by the blast of the incendiary grenade are set on fire instead of being staggered.

When The Tank is shot with incendiary ammunition it is only ignited for 5 seconds, unlike when lit by other sources of fire such as molotovs and gas cans. This causes minimal fire damage. It can be useful to have one player only shoot at the tank in short bursts so that he is lit all the time and cause fire damage while the rest of the team unloads to cause bullet damage and slow him down. Oddly enough however, Tanks hit with incendiary grenades from the grenade launcher will remain on fire until they die.

When shooting an alarmed car with incendiary ammunition, the alarm will not go off. The only exception is with the Grenade Launcher.
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Explosive Ammunition
"I'm gonna grab some of these frag rounds!"

- Ellis

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 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.

Achievements
See main article: Achievements