What is primarily the purpose of the magazine in a repeating firearm?

The primary purpose of the magazine in a repeating firearm is to provide storage for ammunition. The magazine allows multiple rounds to be stored in a compact manner and fed into the chamber of the firearm as needed, facilitating rapid firing without the need to reload after each shot. This design enhances the efficiency of the firearm, enabling shooters to maintain their focus on aiming and shooting rather than the interruption of reloading.

Each of the other choices highlights different aspects of firearm operation or mechanics but does not directly address the main function of the magazine itself. For example, sealing a cartridge is typically associated with the chamber or the cartridge design, not the magazine. Improving trigger performance relates to the firearm's action and mechanics but does not involve the magazine. Supporting the butt during shooting refers to the stock of the firearm, which affects stability and comfort, rather than serving as an ammunition storage solution. Therefore, the correct understanding of a magazine's role is fundamentally about storing and providing timely access to ammunition for repetitive firing.