I have recently come into possession of some unfired, straight-wall rimfire cases I thought were .22 Win Rimfire Magnum, but after (purchasing and) measuring, they turned out to be 5mm/20 cal. and very loose in a .22 mag chamber. They are marked with the Winchester Super X head stamp. Is anyone familiar with this cartridge? Was it a failed experiment? To my knowledge, the cartridge never came to market. Does anyone have chamber dimensions? Can send cartridge dimensions on request. Thanks to all who can help!