The primer is a small charge of impact-sensitive explosive that may be located at the center of the case head (centerfire ammunition) or at its rim (rimfire ammunition). When struck by a firing pin, it will detonate, setting off the propellant and firing the projectile.

Primer unlocking has been used in various firearms, notably the M1 Garand prototypes and the Postnikov APT.

Community content is available under CC-BY-SA unless otherwise noted.