Which element is responsible for starting the firing sequence in a firearm?

The primer is the critical component that initiates the firing sequence in a firearm. When the firearm's firing pin strikes the primer, it creates a small explosion that ignites the propellant powder located inside the cartridge. This ignition produces gas that propels the bullet down the barrel and out of the firearm.

While the bullet, powder, and casing are essential components of a cartridge, they cannot initiate the firing sequence on their own. The bullet is the projectile that exits the firearm and is propelled by gases generated from the powder when ignited. The powder serves as the propellant that provides the energy needed to fire the bullet, but it requires a source of ignition to burn effectively. The casing holds all of these components together and provides a means to chamber and extract the cartridge. However, it does not play a role in starting the firing sequence itself. Thus, the primer is the element that specifically starts the entire process of firing in a firearm.