The 5mm Remington (5mm Remington Rimfire Magnum) is a .20-caliber (.2045-inch) rimfire cartridge introduced by Remington in 1970. Based on a blank power tool cartridge case, its rim diameter of .3252-inch is larger than the 22 WMR case, so manufacturers other than Remington were reluctant to adopt it. Performance is very good, with a 38-grain bullet at 2105 fps...this slightly exceeds the 22 WMR, which was probably Remington's goal, but it was not popular and ammunition was unavailable for many years until CBC brought it back.
