Magazine
Also: Mag
The spring-fed container that holds cartridges and pushes them up to be stripped into the chamber. Either fixed in the rifle or a detachable box.
A magazine stores cartridges under spring tension and presents the top round for the bolt to strip forward into the chamber. It is either a fixed internal box closed by a floor plate or a detachable box that drops free for a fast reload.
For precision shooters the magazine sets a hard limit you have to design around: its internal length caps how long you can load a cartridge. A round seated out to touch the rifling may not fit the magazine, forcing a choice between magazine feeding and the cartridge overall length you want. Detachable systems such as AICS-pattern boxes are popular partly because they offer a little more length.