Which ammunition component actually starts the chain reaction to fire the round?

The component that initiates the chain reaction to fire the round is the primer. When the firing pin strikes the primer during the ignition sequence, it creates a small explosion that generates a significant amount of heat and gas. This reaction is crucial because it ignites the propellant within the cartridge case.

The primer contains a mixture of chemicals that react explosively when struck, producing a flame that travels through a flash hole in the cartridge case and ignites the propellant. This process is essential for the functioning of ammunition as it is the first step in the firing sequence, leading to the subsequent expansion of gases from the burning propellant, which ultimately pushes the projectile out of the barrel.

Other components play important roles in the ammunition's overall function, but they do not initiate the firing sequence. The cartridge case contains the primer and propellant, the propellant serves as the fuel for combustion, and the fuze is used for projectiles designed to detonate at certain times or conditions but does not participate in the initial firing of a round from the firearm.