What must happen for a shotgun shell to be fired?

For a shotgun shell to be fired, the firing pin must strike the primer. This is the critical action that initiates the firing sequence. When the trigger is pulled, it releases the hammer or striker mechanism, which then drives the firing pin forward. This pin strikes the primer located at the base of the shotgun shell. The primer contains a sensitive explosive compound that ignites upon impact, causing a small explosion that ignites the main powder charge within the shell. This rapidly expanding gas then propels the shot through the barrel.

Other components of the firearm, such as the action being open or the shell being removed from the chamber, are unrelated to the actual firing process. The cleanliness of the barrel, while important for optimal performance and safety, does not directly influence the immediate action that causes the shell to be fired. Thus, the essential element in firing a shotgun shell is the successful impact of the firing pin on the primer.