A magazine for a firearm includes an elongated containing body and a closure member adapted to hold cartridges. The closure member longitudinally slides on the containing body and is extendable to open a side of the containing body for cartridge loading. The closure member has opposing sidewalls forming an integral handle surface that can be grasped for moving it. A follower and spring bias cartridges toward one end of the magazine. A latching mechanism is provided for holding the follower in a disengaged position when the closure member is extended, and is configured to release when the closure member is closed. Preferably, the containing body and closure member are constructed to include integrally formed features that cooperate with an automatic latching mechanism, thus minimizing parts.