This article describes the mechanism of atmospheric corrosion as it applies to fired bullets and cartridge cases exposed to the elements. Factors determining local atmospheric corrosivity and the assessment of corrosivity according to ISO standards 9223, 9224, 9226, 8407 and 8565 are explained. The results of an 18 month atmospheric corrosion (aging) observational study of six different ammunition samples, in a low to medium corrosive atmosphere, are presented. The annexed information following this article provide a guideline for determining the general corrosivity of a given atmosphere and pictures of six ammunition samples, taken at different intervals, depict the advancing process of corrosion. Together, this article and the annexed information can serve as a reference for rough estimations of the time a bullet or cartridge case was exposed to the atmosphere. The article can also be used as a guide for further aging studies on ammunition samples and to debunk unprofessional claims about age determination, based on corrosion assessment.