Annulus - A tiny circular groove formed by the junction of the primer and the base of a cartridge case's primer pocket.
Recoil from fired cartridges soon and invariably impress a discernable ring on the breech or bolt face of a firearm from the annulus of the primer, providing some evidence of the amount of use it has seen.
Breech face of a shotgun having seen virtually no use.
Another shotgun with annulus visible around the firing pins; showing the effects of rather more use.
Annulus marked on the breech face of a Webley Revolver
Annulus mark on a rifle's bolt face showing not much use.
And, even less use.
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