Ammunition in DayZ Standalone refers any item or collection of items that may be used with/in a weapon or firearm, this usually refers to cartridges, projectiles, explosives or anything used in similar nature that may cause damage or injury to one's self or others.
Firearms only accept certain calibers (or sizes) of ammunition. A firearms caliber is determined by the barrel diameter which the firearm is chambered in.
Different types of ammunition can be found throughout Dayz, typically inside various buildings and commonly found inside military installations. They may be found as boxed or un-boxed ammunition, in magazines, or sometimes inside Ammunition Boxes.
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