What is the purpose of the rim on a cartridge case?

The rim on a cartridge case serves several important functions, and one of its primary roles is to prevent the case from moving too far into the chamber of a firearm. When a cartridge is chambered, the rim of the cartridge case stops against the rear of the chamber, ensuring that the cartridge is positioned correctly for firing. This stopping point is crucial for maintaining the integrity of the firearm's operation, as it helps align the cartridge with the chamber and the barrel.

Understanding the other options allows for a deeper insight into the rim's specific functions. While the bullet must be held securely in place, the bullet is primarily retained by the neck of the case rather than the rim. Additionally, the primer, which ignites the gun powder, is housed within the base of the cartridge case, not inside the rim itself. Lastly, while a good seal during firing is important for effective operation, that task is primarily fulfilled by the case mouth against the chamber walls, not specifically by the rim.