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There are various types of ammunition featured in the Counter-Strike series, all of which have their own unique properties (damage, armor penetration, cost, etc.) which in turn also effects the efficiency of the weapons that fire them.
Broadly, ammunition can be divided into pistol, rifle, shotgun, and heavy (Counter-Strike: Condition Zero Deleted Scenes only) categories. While there are many types of pistol and rifle ammunition, the only shotgun and heavy ammunition available are the 12 Gauge shotshells and 66mm respectively.
The simplest way to gauge the power of any particular ammunition is to observe the amount of energy it generates (in Joules) as stated in the VGUI menus.
From Counter-Strike to Counter-Strike: Condition Zero, ammunition needs to be purchased by players manually in the buy menu with money, adding another layer into the game's economy management.
In Counter-Strike: Source, players no longer need to purchase ammunition. Players automatically receive full ammo (loaded and backup) for all their weapons when a new round starts. Players will also always receive ammunition immediately after picking up a weapon, unless it was already empty. When out of ammo, running back to the buy zone and purchasing ammo for a primary weapon will get you another supply of ammunition.
If the player drops a weapon, its reserve ammo will also be discarded alongside with the weapon itself, unlike previous games where the player can keep the ammo of that weapon even when dropping it.
However, should the player wields a primary and secondary weapon that shares the same ammo, the player can keep the reserve ammo.
Prior to Global Offensive, if a player equips a primary weapon that shares ammo with his secondary weapon (MP5 with Glock 18 for example), the player will draw ammo from a single ammo pool. While this can make it easier to find ammo (in older games), the player's reserve rounds can easily deplete if ammo conservation is not taken into account.
In Deleted Scenes, because the player can equip multiple primary weapons and secondary weapons, different weapons using the same ammo will thus also share one ammo pool.
Prior to Global Offensive, machine guns use separate ammunition pools despite sharing a caliber with other weapons. The M249 doesn't share ammo with other weapons in 5.56x45mm NATO, and the M60 doesn't share ammo with 7.62x51mm NATO weapons.
In Deleted Scenes, machine guns require different ammo boxes to resupply their ammo.
In Counter-Strike: Global Offensive ammo sharing is completely removed. All weapons come with their maximum ammunition when purchased and are refilled when the next round starts (like Source). Additional ammunition cannot be bought once a round starts.
The caliber of a weapon can be viewed in the buy menu until Global Offensive. However, Global Offensive still includes references to weapon calibers in game files.
In older betas, there was no option to buy primary and secondary ammo. Instead, a player had to bring up a menu and purchase the according ammunition for a weapon (example, 9mm for the Glock). As such, it was possible to buy ammo for other team members once after the reserve ammo was full (as ammunition had world models borrowed from Half-Life and would remain if not taken).
The rifles that use 5.56 ammunition read 5.56 NATO while the M249 reads 5.56 Parabellum. This may have been done to differentiate the assault rifles' ammo and the M249's exclusive 200 round boxes.
Early betas do not differentiate these two types of ammunition, which means that 5.56mm rifles can carry up to 200 rounds in reserve. This was fixed in beta 6.0.
↑"Fixed a few cases where reserve ammo was shared between multiple weapons. All weapons now individually handle their own reserve ammo." April 28, 2015 update notes.