High Quality Content by WIKIPEDIA articles! The .300 Remington Ultra Magnum, also known as the .300 Ultra Mag` or .300 RUM is a 7.62 mm (.308in.) caliber 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 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.