What causes the projectile to exit the barrel of a firearm?

The projectile exits the barrel of a firearm primarily due to the pressure generated by ignited gunpowder within the cartridge. When the firing pin strikes the primer, it ignites the gunpowder, causing a rapid expansion of gases. This high-pressure gas pushes against the base of the bullet, propelling it down the barrel and out of the firearm.

Other options do not directly cause the projectile to exit the barrel. The magazine release is used to remove the magazine from the firearm, and while a manual trigger pull is necessary to initiate the firing process, it is the combustion of gunpowder that produces the force needed to propel the bullet. The weight of the bullet is also irrelevant to the mechanism of its exit; it influences factors like accuracy and velocity but not the propulsion provided by gas pressure. Therefore, pressure from ignited gunpowder is the key driver that enables the bullet to travel out of the barrel.