What purpose does the case of a rifle or handgun cartridge serve?

The case of a rifle or handgun cartridge serves the essential function of holding all of its components together. This includes the primer, powder, and bullet, which are crucial for the cartridge to operate effectively when fired. The casing provides the structure that supports the internal components under pressure and ensures that they are properly aligned when the firing pin strikes the primer. This containment is critical for creating a controlled explosion that propels the bullet forward.

While the other options mention aspects related to firearms or ammunition, they do not accurately describe the primary function of the cartridge casing. Increasing shot accuracy typically relates more to factors like the barrel and bullet design rather than the casing itself. Making reloading easier involves additional techniques and tools, rather than being a direct purpose of the casing. Preventing rusting of the bullet can be a factor in ammunition design, but again, it does not reflect the main purpose of the case. The primary role of the cartridge case is indeed to hold all components securely together.