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The Ultimate Reloading Manual
Wolfe Publishing Group
  • reloading manual
  • alliant reloading data
  • reloading brass
  • shotshell reloading
  • bullet reloading
The Ultimate Reloading Manual
hodgdon load data

.300 Remington Ultra Magnum

Author: Brian Pearce
Date: Apr 15 2020

Thirty-caliber magnum cartridges have been steadily gaining in popularity since the introduction of the .300 H&H Magnum back in 1920, and are often touted as all-around cartridges for all game in North America, African plains game and open country hunting. Other notables include the .300 Weatherby Magnum, .300 Winchester Magnum and .300 WSM, which have enjoyed widespread popularity among hunters, law enforcement and military. Remington wanted its own, and in 1999 offered the .300 Remington Ultra Mag, based on a modified version of the large .404 Jeffery case. It is beltless with a slightly rebated rim and features a large powder capacity that holds in excess of 100 grains with select powders, which is notably greater than the big .300 Weatherby Magnum. Advertised factory load ballistics include a 150 grain bullet at 3,450 fps, a 180 at 3,250 fps or a 200 at 3,032 fps from a 24-inch barrel. In an effort to add versatility to the cartridge and offer reduced recoil loads, a few years back Remington added three Power Level options. Option I is ballistically similar to the .30-06, Power Level II resembles the .300 Winchester Magnum, while Power Level III is a full house load offering the previously mentioned velocities.

Due to the cartridge’s powder capacity, Large Rifle Magnum primers are essential to achieve reliable ignition, with Remington 9 ½ Magnums used to develop the accompanying data. Federal 215 or CCI 250 primers can be substituted without significant changes in velocities or pressures.

Never reduce starting loads, or erratic velocities and pressures can occur. In some instances, such as with Accurate Magpro, a “ball” powder, charges lighter than listed resulted in dangerous hang fires. Many of the starting loads utilizing slow burning extruded powders can duplicate Remington Power Level I and II factory loads. Accuracy testing, and checking for powder position sensitivity, will help in determining the best load for your rifle.

Handloading the .300 RUM posed no difficulties; however, like the .338 RUM, I experienced an unusual occurrence with cases that had either been fired with maximum loads, or had been reloaded several times. As the primer pocket loosened (holding the primer less securely), cartridges in the magazine exposed to the quick and heavy recoil associated with full-power loads would occasionally have the primer jolted out. The prevention is to use new or once-fired cases, especially when in the field, or when ammunition must be ultra reliable.

The industry pressure limit for the .300 RUM is 65,000 psi, with all accompanying loads being within that limit.