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

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.

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 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.
 * 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 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.

Incendiary Ammunition


The Incendiary Ammunition is a special type of ammo that can be found in Left 4 Dead 2 randomly throughout the campaigns. After picking up this type of ammo, all gun shots for Tier 1 and Tier 2 weapons are flammable and can set Infected on fire (with some exceptions). Infected in hazmat suits are invulnerable to incendiary ammo. The only evidence of this at the moment is a video that can be found here showing up at about 5:49 through 6:49. The video shows an incendiary ammo pile and the effects.

Explosive Ammunition
The Explosive Ammunition is a second type of special ammo that is found in Left 4 Dead 2. This makes the bullets explosive, instantly killing the infected and knocking down anything close enough to it. It is possible that the Riot Gear infected are immune to this.

Ammo Packs
Ammo Packs appear as medkit replacement in Left 4 Dead 2. The Ammo Pack was mentioned in Kotaku's "Hands-on Impressions" article. It is a one-time-use ammo refiller. It uses the health pack's slot, so you must choose whether to carry a health pack or ammo pack.

Many feel that the ammo packs will be essentially useless should the pistols remain with unlimited ammo capabilities. Many would rather keep their first aid kit and switch to pistols once they're out of ammo, as they had to in the first game, or even take one of the new Close Combat Weapons. However, as Left 4 Dead 2 is looking somewhat more intense than the previous game, it may still be a good idea to grab an ammo pack, so long as health isn't an immediate issue. However, 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.