In the early 1980s Aubrey White and Noburo Uno of North American Shooting Systems (NASS) based in British Columbia Canada began experimenting with the full length .404 Jeffery by reducing the taper and necking it down to various calibers such as 7 mm, .308, 311, 338, 9.3 mm and .375. These cartridges were known variously as the Canadian Magnum or the Imperial Magnums. Rifles were built on Remington Model 700 Long Actions and used Macmillan stocks. Cartridges were fire formed from .404 Jeffery cases with the rim turned down, taper reduced and featured sharp shoulders.

Both Remington and Dakota Arms purchased the formed brass designed by Noburo Uno for use in their own experimentation and cartridge development. In 1999 Remington released the first of a series of cartridges virtually identical to the Canadian Magnum cartridges which featured a slightly wider body, increased taper, and shallower shoulders and named it the .300 Remington Ultra Magnum. Dakota too released their own version of the cartridge but chose not to turn down the rim and shortened the case to work in a standard length action. Remington would go on to design their own shortened versions of the Ultra Magnum cartridge which they were to call the Remington Short Action Ultra Magnum or RSAUM for short.

The history of the Imperial Magnums are also discussed in detail in IAA Journal 430, (March/April 2003)