Wiktionary
n. (context military firearms English) The identification marking on the base (bottom) of a cartridge casing.
Wikipedia
A headstamp is the markings on the bottom of a cartridge case designed for a firearm. It usually tells who manufactured the case. If it is a civilian case it often also tells the caliber: if it is military, the year of manufacture is often added.
The headstamp is punched into the base of the cartridge during manufacture. A resource for identifying where the ammunition originated can be found at Cartridge Collectors.
Usage examples of "headstamp".
There was no need to ask if he knew how to sort by cartridge size, by whether the case was necked and how much, by the type of bullet seated in the case, by the headstamp on the base of the cartridge, and whether it was rimmed or rimless.