The .300 Remington Ultra Magnum was introduced by Remington in 2000. It is reminiscent of the 300 Canadian magnum (which was derived from the 404 Jeffery). Ballistically, the 300 Remington Ultra Magnum is not too far behind the 30-378. The case size has two significant advantages over the various wildcat and proprietary numbers based upon the 416 Rigby case; adoption to Remington (and similar) bolt-action rifles is much simpler and this case does not overstress the typical bolt-action receiver. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.