The Imperial Magnums were made in 7mm, .300, .311, .338 and .360. The .311 was .303 British diameter and Speer made a 200 grain Grand Slam for a short time. The .360 used .358" bullets. The Remington Ultra Magnums are a shameless knock-off of the Imperial Magnums and there's hardly any difference in performance. I've got all of them in my cartridge collection and I can measure the cases if you want. Later, the name were changed to Canadian Magnum and a .375 added. There's reloading data for the 7mm, .300, .338 and .375 Canadian Magnums in Cartridges of the World, 9th Edition.