Final answer:
The firing pin of a firearm ignites the primer powder mixture, creating a high-pressure gas that propels the bullet out of the barrel.
Step-by-step explanation:
When you pull the trigger, the firing pin of the firearm hits the base of the cartridge, igniting the primer powder mixture. This initiates the combustion process and creates a flame that travels through the flash hole in the cartridge case, reaching the main gunpowder charge. The rapid combustion of the gunpowder generates a high-pressure gas, which propels the bullet out of the barrel and towards the target.