The .300 Super Magnums (.300 Rem. Ultra Mag and .30-378 Weatherby Mag.)

By Chuck Hawks

In recent years there has been a trend among wildcat cartridge aficionados toward outsized "super" .300 magnums. In 1996 Weatherby responded to this trend by chambering their Mark V rifle for the huge .30-378 Belted Magnum. In 1999 Remington brought out the beltless .300 Ultra Mag in their Model 700 rifle.

These are the two most over-the-top .300 Magnums ever introduced by major rifle and ammunition companies. Both are based on elephant rifle cartridge cases necked down to accept standard .308" diameter bullets. Needless to say, they are at their best with very slow burning powders and heavy bullets.

A brief summary of the two most popular .300 Super Magnums follows. For more extensive coverage of these and other potent .300's, such as the .300 Dakota, see the individual cartridge articles on the Rifle Cartridge Page.

.300 Remington Ultra Mag

The .300 Remington Ultra Mag is loosely based on a blown out and necked down .404 Jeffery case with a 30 degree shoulder. This monster has 20% more powder capacity than the .300 Win. Mag. and 13% more capacity than the .300 Weatherby Mag.

The .300 Ultra has a rebated rim that allows the body of the case to be fatter than the rim, which is the same size as the rim on the .300 Winchester Magnum and other standard magnum cases. This allows it to work with standard diameter magnum bolt faces, at the price of potentially reduced feeding reliability. No cartridge with a rebated rim is a good candidate for use on dangerous game, where a failure to feed could be fatal.

.300 Ultra Mag rifle barrels should be at least 26 inches long or the performance falls back to standard length .300 Magnum levels. 28 or 30 inch barrels are even better.

American Rifleman magazine reported that their Model 700 test rifle in .300 Ultra Mag with a 4-12 power scope, sling, and 4 rounds of ammunition weighed 8.25 pounds. An 8.25 pound .300 Ultra Mag rifle slams the shooter's shoulder with 38.4 ft. lbs. of recoil energy.

This is totally unacceptable recoil for most shooters, which Remington has tacitly admitted by introducing "Power Level" ammunition for their Ultra Mag cartridges. In the case of the .300 Ultra Mag, this means reduced power factory loads that duplicate the ballistics of the .30-06 (Level 1), .300 Win. Mag. (Level 2) and full power loads (Level 3). Note that Remington claims duplicating the performance, not the recoil, of those other .30 caliber cartridges. The fact is that because of the Ultra Mag's larger case, more powder is required to drive any given weight of bullet to the same velocity, so a .300 Ultra Mag load that duplicates .30-06 performance kicks harder than a real .30-06.

Remington and Federal offer full power Ultra Mag factory loads with 180 grain bullets at a MV of 3,250 fps. Remington also offers a 150 grain bullet at a MV of 3450 fps with ME of 3964 ft. lbs. This bullet seems pretty light for such a big case.

Better is the load Remington offers with a 200 grain Nosler Partition bullet (SD .301) at a MV of 3,025 fps and ME of 4063 ft. lbs. At 200 yards the figures are 2636 fps and 3086 ft. lbs. This is the factory load to choose for Alaskan moose and other very large game. The trajectory of that load is as follows (Remington figures): +2.4" at 100 yards, +2" at 200 yards, 0 at 250 yards, -3.4" at 300 yards, and -14.6" at 400 yards.

The handloader can use the entire range of .30 caliber bullet weights in the .300 Ultra Mag, and can duplicate the factory loads with heavy charges of appropriate slow buring powders. No doubt the all-around bullet weight of 180 grains will prove to be just as popular with reloaders just as it has with those who buy factory loads.

.30-378 Weatherby Magnum

The Weatherby .30-378 Magnum is another attempt at a super .300 Magnum. It is based on the belted .378 Wby. Mag. case necked down to .30 caliber. The .378 case looks much like a belted version of the .416 Rigby case, on which it was based.

Unlike the .300 Ultra Mag, the .30-378 Weatherby has a full diameter rim for maximum feeding reliability from the box magazines of bolt action rifles. It is suitable for use on thin-skinned dangerous game of the largest sort, such as lion, tiger, grizzly, brown, and polar bears.

Weatherby Mark V rifles for this caliber weigh about 8.5 pounds (empty and without a scope or sling) and have 28" barrels. Needless to say, a bolt action rifle with a 28" barrel is not a real handy weapon, but the long barrel and substantial weight is a practical necessity for a rifle of this power. Even in a scoped, fully loaded, Mark V rifle weighing 9.75 pounds the 180 grain factory load for the .30-378 punishes the shooter with some 51.4 ft. lbs. of recoil energy.

Weatherby factory ballistics for the .30-378 call for a 165 grain bullet (SD .248) at a MV of 3500 fps with ME of 4488 ft. lbs., a 180 grain bullet (SD .271) at a MV of 3,420 fps with ME of 4676 ft. lbs., and a 200 grain bullet (SD .301) at a MV of 3,160 fps with ME of 4434 ft. lbs. Weatherby ballistics tables show the following trajectory for the 200 grain Nosler Partition bullet: +3.2" at 100 yards, +3.9" at 200 yards, 0 at 300 yards, and -9.1" at 400 yards.

Even at 400 yards the remaining energy of this load is a whopping 2541 ft. lbs. For the shooter who can reliably deliver solid hits in the heart/lung area at that range (probably less than 1 in 1000 hunters), the .30-378 undeniably has the power to kill elk and moose at 400 yards.

It would seem that heavy bullets of 200 to 250 grains would be most suitable for such an outsize case, but no bullets over 200 grains are available in factory loads. However, the reloader has access to the various 220 grain bullets (SD .331), and the super heavy Barnes 250 grain Original bullet (SD .376).

Hodgdon Powder Company reloading data shows that the 220 grain Nosler Partition bullet can be driven to a MV of 3180 fps in the 28" barrel of a Weatherby Mark V rifle. With a well placed bullet this should do for any animal on earth, although no .30 caliber cartridge it really the best choice for thick-skinned dangerous game.


The .300 Ultra and .30-378 Weatherby super magnums are the hardest hitting of all the popular ultra-long range rifle cartridges. That is true at both ends, of course!

Case life and barrel life are expected to be short for both the .300 Ultra and the .30-378 because of the high velocity and the great amount of powder burned per shot. Recoil is severe, particularly in the relatively lightweight factory rifles often chambered for these cartridges. However, for the shooter who wants to create "cartridge envy" in the minds of his or her peers, the .300 Remington Ultra Mag and .30-378 Weatherby Magnum are hard to beat.

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Copyright 2002, 2016 by Chuck Hawks. All rights reserved.