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 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 do this if you are close.
 * It is rarely a good decision to run back through half the map just for more ammo, as the Director will probably spawn multiple hordes and Special Infected to and from the ammo pile. Use your pistols or melee weapon until you reach the next ammo pile.
 * In Crescendo Events, players should try to stock on ammo before the crescendo begins.
 * In finales, try to reach the ammo pile before the Tank arrives.
 * Ammo is usually found near Crescendo Events and on longer levels.
 * If you are concerned about your ammo, develop a habit of switching to your pistols or melee weapon often.
 * The Ammo Piles cannot refill ammunition for the Grenade Launcher or M60 so try to preserve as much as the ammo as possible for Special Infected or hordes.
 * Be aware: In Left 4 Dead, ammo piles and cans will restock the maximum possible in reserve and not account for the missing ammo in your gun, potentially requiring a pickup, then a reload, and then another pickup to gain full ammunition. In Left 4 Dead 2, ammo piles will restock the maximum possible in reserve and temporarily add the amount your are missing in-gun to your reserve, meaning that a single pickup and reload will result in maximum ammo.
 * Using a Combat Shotgun as an example, it holds 10 shells and 90 in reserve. If you have 1 shell loaded and 75 in reserve, the ammo pile will give you 99 reserve shells, topping off the shotgun even though it hasn't been reloaded yet.

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; 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. Never fire while moving as accuracy is greatly reduced with motion. Use Pistols for run and gun situations. 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 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.
 * 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 its munitions 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

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.