Which ammunition round component expands in and seals the barrel's chamber to keep gas from escaping to the rear when the cartridge is fired?

The cartridge case plays a critical role in the functioning of ammunition. When a cartridge is fired, the cartridge case expands due to the high pressure generated by the burning propellant. This expansion effectively seals the chamber of the firearm. By creating a tight fit against the chamber walls, the cartridge case prevents the escaping gases from moving backward into the firearm. This sealing is essential because it helps maintain the pressure needed to propel the projectile forward and ensures safe and effective operation of the weapon.

The other components, while important, do not serve the same primary function. The projectile is what is fired from the barrel, the fuze is used in certain types of ammunition for detonation purposes, and the primer is responsible for igniting the propellant but does not contribute to sealing the chamber. Thus, the cartridge case stands out as the component specifically designed to expand and secure the chamber when a round is fired.