A magazine for storing cased telescoped (CT) cartridges and delivering the CT cartridges to a firearm configured to fire CT cartridges. The magazine includes a fixed lip and a moveable lip. The fixed lip and the moveable lip of the magazine are configured to prevent a CT cartridge located in a top-most position within the magazine from being pushed out a top of the magazine when the magazine is not fully inserted into a magazine well of the firearm configured to fire CT cartridges. The moveable lip may pivot laterally on a pivot point and move out of the way of the CT cartridge located in the top-most position within the magazine when the magazine is fully inserted and locked into the magazine well of the firearm configured to fire CT cartridges.