What is the purpose of a primer in ammunition?

The purpose of a primer in ammunition is to ignite the propellant or powder charge. When the firing pin strikes the primer, it creates a small explosion that ignites the powder inside the cartridge case. This ignition is crucial for the functioning of firearms, as it generates the high-pressure gas needed to propel the bullet out of the barrel. The primer is a vital component of the ammunition, as it ensures that the firing process occurs reliably and efficiently.

The other options do not accurately describe the function of a primer. For instance, measuring the size of the bullet pertains to cartridge dimensions and design rather than the primer's role. Sealing and confining gases typically refers to the case itself and how it interacts with the bullet to ensure that energy is directed forward and does not escape. Acting as a projectile is unrelated to the primer, as the projectile is the bullet, which is a separate element of ammunition that is propelled by the force generated from the ignited powder. Understanding the function of different components in ammunition is important for comprehending how a firearm operates.