The .300 Remington Ultra Magnum, also known as the .300 Ultra Mag' or .300 RUM is a 7.62 mm (.308in.) calibre rifle cartridge , 7.62x72mm, or .30 caliber 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 an original designed beltless, rebated rim cartridge, capable of handling most large North American game, as well as long-range shooting. The .300 Remington Ultra Magnum is second only to the .30-378 Weatherby Magnum in cartridge-case capacity.
In 1999 Remington jumped on the current high performance bandwagon by creating their own beltless magnum. The .300 RUM was the first in a family of new Ultra magnum cartridges based on the .404 Jeffery, later joined by the 7 mm RUM, .338 RUM, and .375 RUM. In addition, Remington later created a shortened version called the Remington Short Action Ultra Magnums to compete with Winchester's line of Winchester Short Magnums.
It is more efficient than the .30-378 Weatherby Magnum in that it can produce nearly as much energy and velocity with the use of roughly 10 percent less powder. Note the .30-378 weatherby headspaces off of the belt not the shoulder.
The cartridge produced significantly more energy in testing than the .300 Winchester Magnum and .300 Weatherby Magnum cartridges did.
Because the .300 RUM is beltless, it headspaces on the shoulder giving it a potential edge in accuracy over belted magnums, in addition to longer case life. The rebated rim allows this cartridge to be used in an action with a magnum bolt faces (.532 inches).
To further enhance the usability of guns chambered in this cartridge, Remington now offers three different power levels. Power level 1 cartridges are similar in performance to the .30-'06 and are offered in three different 150-grain (9.7 g) bullet selections. The power level 2 performs similar to the .300 Winchester Magnum and is offered in two different 180-grain (12 g) bullet selections. Finally, the power level three ammunition is offered in 150 to 200-grain (13 g) bullet selections. These bullets exit the muzzle at 3,032 feet per second (924 m/s) for the 200-grain (13 g) variety and the 150-grain (9.7 g) bullet is sent along at 3,450 ft/s (1,050 m/s).