Which part of ammunition ignites the powder when the trigger is pulled?

The primer is the component of ammunition that ignites the gunpowder when the trigger is pulled. When the firearm's trigger is pulled, it typically causes a hammer or firing pin to strike the primer. The primer contains a small amount of explosive material that, when struck, creates a small explosion. This explosion generates enough heat to ignite the gunpowder (or propellant) contained within the casing of the cartridge.

While the casing serves as the container for all the components of the cartridge, and the bullet is the projectile that leaves the barrel, neither plays a role in igniting the powder. The powder, on its own, requires ignition, which is provided by the primer. Thus, the primer is the critical component that initiates the firing process in a cartridge.