.300 Rem. Ultra Mag

The .300 Remington Ultra Magnum, often abbreviated as .300 RUM, was introduced by Remington Arms in 1999 as part of their Ultra Magnum family of cartridges. Developed in response to the growing demand for long-range, high-velocity magnum cartridges, the .300 RUM is based on the .404 Jeffery case, which was necked down to accept .308 caliber bullets. This parent case, combined with a beltless and rebated-rim design, allows for greater powder capacity and efficiency compared to traditional belted magnums, such as the venerable .300 Winchester Magnum. The cigar-sized case was engineered to push the performance envelope, making the .300 RUM one of the fastest and flattest-shooting .30 caliber cartridges available at the time of its release. As reloading technology and bullet design progressed, the .300 RUM quickly gained attention among precision shooters, reloaders, and big game hunters.

The .300 Remington Ultra Magnum’s primary application is long-range hunting and competitive shooting where downrange energy and velocity are at a premium. With its substantial case volume, the cartridge can deliver high muzzle velocities that equate to flatter trajectories and less wind drift compared to other .30 caliber magnums. This makes it particularly attractive for shooters engaging targets or game animals at extended distances. For reloaders, the cartridge's potential is unlocked through careful powder selection and attention to detail in case preparation; consistent primer seating, precise charge weights, and high-quality projectiles are essential to harness its full accuracy and power. The .300 RUM does generate significant recoil and muzzle blast—factors that must be considered when choosing a suitable rifle platform, optics, and shooting technique.

Target species for the .300 Remington Ultra Magnum tend to be larger North American and African big game. Its sheer power is more than adequate for elk, moose, bear, and even bison, especially when using premium, controlled-expansion bullets that retain weight and penetrate deeply. Some hunters also use the cartridge for long-range deer or antelope hunting, but its power is often considered overkill for these lighter species. The .300 RUM’s ability to deliver decisive terminal performance on large, tough animals is a major selling point for those venturing into challenging environments or pursuing game at long distances where fast, ethical kills are paramount.

When it comes to bullet weights and other caliber characteristics, the .300 Remington Ultra Magnum is typically loaded with projectiles ranging from 150 to 220 grains, though 180 to 200 grains are most commonly used for big game applications. The high case capacity allows reloaders to experiment with slow-burning powders, such as Retumbo or H1000, to maximize velocity and stabilize heavy, high-ballistic-coefficient bullets. Typical factory muzzle velocities reach 3,250 fps with a 180-grain bullet and can exceed 3,400 fps with lighter projectiles. The cartridge’s freebore and throat design can impact seating depth, so precise measurement and case trimming are critical during the reloading process. The .300 RUM’s reputation for exceptional downrange energy, impressive accuracy potential, and excellent reloading flexibility makes it a favorite among reloaders who demand maximum performance from their .30 caliber magnums.