From Russ Chastain,
Your Guide to Hunting / Shooting.
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Definition: A device for igniting the powder charge in a modern cartridge of ammunition. Round in shape, and inserted into a pocket built into the center of the base (or head) of a cartridge case in centerfire rifle, handgun, and shotgun ammunition. When struck in the center by a firing pin, the primer produces a spark or flame within the cartridge, which ignites the powder charge, thus driving the bullet or shot downrange.

Primer may also refer to the priming compound in the rim of rimfire ammunition, which serves the same purpose as a centerfire primer (igniting the powder charge). This is in the minority, however, and in shooting texts primer most often refers to a centerfire primer as described above.

Pronunciation: pry mer (noun)

Examples: The firing pin struck the primer, igniting the powder charge.