.300 Remington Ultra Magnum, also known as the .300 Ultra Mag, 7.62×72mm or .300 RUM is a 7.62 mm (.308 inch) rifle cartridge introduced by Remington Arms in 1999. The .300 Remington Ultra Magnum is one of the largest commercially available .30 caliber magnums currently being produced. It is a beltless, rebated rim cartridge, capable of handling all large North American game, as well as long-range shooting. Among commercially produced .30-caliber rifle chamberings, the .300 Remington Ultra Magnum is second only to the .30-378 Weatherby Magnum in cartridge-case capacity.
Origin & History
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 McMillan 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.
Design & Specification
.300 Remington Ultra Magnum is a member of the Remington Ultra Magnum cartridge family based on the .404 Jeffery via the Canadian Magnum cartridges. As these cartridges feature wider bodies than the belted magnums based on the .375 H&H case, these cartridges have greater case capacities than their corresponding full length belted Magnum cartridges such as the 7 mm Shooting Times Westerner, the .300 Weatherby Magnum, .340 Weatherby Magnum and the .375 Ackley Improved.
The .300 RUM features a rebated rim much like the .300 Canadian Magnum so as to allow the cartridge to function in the Remington M700 action without having to increase the bolt and bolt face diameter of the action. Unlike the belted Magnum cases based on the .375 H&H Magnum cartridge, the .300 RUM does not have a belt. All things being equal, a beltless cartridge would feed more reliably and more smoothly than a belted cartridge.