The .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.
The .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. Furthermore, as this is a beltless cartridge headspacing is designed to take place on the shoulder, which is considered beneficial by some as it is thought to promote accuracy and prolong case life of the cartridge. As modern belted magnums such as 7mm Remington Magnum and .300 Winchester Magnum actually headspace on the shoulder despite retaining the belt found on their parent cartridge, belted or unbelted is basically irrelevant in discussions of modern cartridges, and particularly so in discussions of unbelted cartridges.
SAAMI recommends that the barrel have a 6 groove contour with a twist rate of one revolution in 10 in (250 mm). The barrel is to have a groove width of .115 in (2.9 mm). Bore is given as .300 in (7.6 mm) and a groove is .308 in (7.8 mm). Maximum case overflow capacity is 122.5 gr. of water (7.30 cm3). SAAMI recommended Maximum Average Pressure is set at 65,000 psi (4,500 bar). Remington's ultra magnum cases were made wider than the .404 Jeffery case by .006 in (0.15 mm). The brass was made thicker so as to withstand the higher pressure of the new cartridge as the Jeffery cartridge had a maximum average pressure rating of 3,650 bar (52,900 psi).
Remington ammunition for the .300 RUM is available in three Power Levels. Power Level I duplicates the .30-06 Springfield, Power Level II that of the .300 Winchester Magnum and Power Level III is the full power load. Remington offers the full power (Power Level III) .300 Remington Ultra Magnum ammunition in 150 gr (9.7 g) at 3,450 ft/s (1,050 m/s), the 180 gr (12 g) at 3,250 ft/s (990 m/s) and the 200 gr (13 g) at 3,032 ft/s (924 m/s). These are among the highest velocities attained by a .30 caliber production rifle cartridge.