Ammo

Ammunition piles (colloquially known as Ammo) are found in Safe Rooms and scattered elsewhere throughout the game. They are used to restock a player's reserve ammunition for primary weapons.

Note that the Grenade Launcher and M60 cannot be resupplied from ammo piles and the various Pistols, the Minigun, and the Heavy Machine Gun have no need of them since all have unlimited ammo.

Ammo comes in two types: A loose pile and a coffee-can-style bucket full. Both types are found in Left 4 Dead; Left 4 Dead 2 only features the loose pile. Their mechanics are identical during game play.

Tactics

 * Ammo piles have unlimited stocks of all types of relevant ammunition.
 * 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 nearby 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, players should try to reach the ammo pile before the Tank arrives.
 * The location of an ammo pile should be factored into defense plans before triggering a finale.
 * Outside of safe rooms, ammo is usually found near the venues for Crescendo Events and at intervals on longer levels.
 * If a player is 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 arguably the easiest weapon to manage as it has only eight shells to keep track off and quick top-up reloads can done on the fly whilst Pistols can be used to engage targets at medium to long ranges with no ammo reserve penalty. Players must bear in mind that this shotgun is devastating at close ranges and that its effectiveness degrades markedly beyond such distances - although distant targets can still be struck (and wounded) by two or three buckshot pellets.  Overall this weapon is a good choice for chapters characterised by plenty of short range targets at close quarters - such as The Apartments in Left 4 Dead. In Left 4 Dead 2, the Pump and Chrome Shotgun have severely reduced ammo reserve capacities and thus require more careful ammunition management and greater use of Pistol back-up.
 * The Auto Shotgun (or Tactical Shotgun in Left 4 Dead 2) is a Tier 2 weapon and is therefore inherently more capable than the generic Tier 1 Pump Shotgun. Experienced users know to fire in a disciplined way at selected targets whilst keeping  an eye on the ammo counter.  Rapid magazine top-ups are essential to avoid the cooldown time-delay imposed if the weapon's magazine is emptied.  For this reason, when hard pressed and caught with an empty Auto Shotgun, it is often better to switch to Pistols and melee in order to continue the fight. This weapon's fully automatic capability is best used when facing Tanks, Witches, and hordes. Holding down the fire key will fire this weapon in full-auto mode, but even this is slower than rapidly hitting the fire key.
 * 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 players. 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 has a high rate of fire and can very quickly deplete an undisciplined player's ammo reserve. If fired in short 3-5 round bursts at medium to long ranges and when crouched, this weapon performs very capably.  (Indeed a review of a player's statistics may reveal that this weapon is most accurate in terms of kills-per-shots-fired.)  Long bursts should be used only against  large hordes, big threats like a Tank or Witch and close encounters with the likes of Smokers and other Uncommon Infected. When facing a horde in a choke point such as a corridor or doorway, players should aim for their heads as this will drop them almost instantly.  Random solo Common Infected can be taken down by the aforementioned short burst delivered to the central body area.  As indicated earlier, it is essential that players keep constant stock of their ammo reserve and plan how best to use those rounds remaining once the count gets down to 50 percent or less.  For example, switching to Dual Pistols and relying more on co-op team partners are good options for a sub-machine gunner anxious about the ammo situation.  In Left 4 Dead 2, the Submachine and Silenced Submachine Gun have roughly a 35% increase of reserve ammunition (480 backup rounds to 650) this naturally increases their effectiveness, though active fire discipline and firing short bursts remain of fundamental importance.
 * 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 are 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 Med kits 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