A cartridge magazine for firearms has an elongated channel to receive a plurality of cartridges. The cartridge magazine has a box forming a housing of the cartridge magazine, a spring operated follower for pressing the cartridges in the magazine to an upper position. In this position, the topmost cartridge is ready for feeding into a chamber of the firearm and in this position the topmost cartridge rests against a pair of retaining lips. The lips remain in a first position when the topmost cartridge is in the upper position and the lips are operatively displaceable to a second position wider than the first position.