A magazine for storing cased telescoped (CT) cartridges. The magazine includes vertically extending side-wall ribs that project inwards into a loading channel. The side-wall ribs are positioned in alignment with a circumferential groove in each CT cartridge. The side-wall ribs prevent a backwards oriented CT cartridge from being pushed from the loading position through the loading channel into a body of the magazine. A moveable lip at the top of the magazine may move out of the way when the magazine is attached to a firearm, to allow the CT cartridge located in the loading position to be fed vertically out of the top of magazine. Each CT cartridge may include a thermal protective insert that provides thermal insulation from heat emanating from a barrel of a firearm when the CT cartridge is located in a chamber of the firearm.