Ammo Brief: .30-378 Weatherby Magnum

by Tommy Grant

A quick look at the .30-378 Weatherby Magnum, a lightweight long-ranger.

The .30-378 Weatherby Magnum is the .378 Weatherby Magnum case necked to .30 caliber. It was created by Roy Weatherby in the 1950s, upon request by the U.S. government, which wanted to determine how well various types of armor plating stood up to penetration by high-speed projectiles.

Using special lightweight bullets made by Speer Bullet Company, Weatherby managed to exceed a muzzle velocity of 6,000 fps during his tests. Years later, while still a wildcat, the .30-378 became popular among members of the Original 1,000-Yard Rifle Club in Pennsylvania, where it was used to shoot the world record group at that distance. After several years of coaxing by firearms writer Layne Simpson, Roy’s son, Ed Weatherby, decided to add the cartridge to the Weatherby lineup in 1996.

During that year, Norma started loading the ammunition for Weatherby, and Weatherby began to offer its Mark V rifle in that chambering. The first rifle was auctioned off with the proceeds going to Ed Weatherby’s favorite charity. The .30-378 went on to become the best-selling cartridge in the Mark V rifle for several years and continues to sell at a steady pace.

General Comments

The .30-378 case can hold more than 120 grains of powder, compared to about 90 grains for the .300 Weatherby. With the advent of slower-burning powders, increased capacity promises a useful advantage to the handloader. Availability of 250-grain match-grade bullets served to increase potential benefit and demand for a chambering with increased powder capacity. The .30-378 Weatherby certainly delivers on this promise.

It’s a simple matter to load 250-grain Sierra MatchKing bullets to produce almost 3,000-fps muzzle velocity, without exceeding .30-06 pressure level—from a 26-inch barrel. Lighter bullets can be driven faster, but with those this chambering offers less advantage over standard .300 Magnum chamberings. When bullets lighter than 200 grains are fired from a 26-inch barrel, this cartridge is only marginally superior to the .300 Weatherby.


Tests have demonstrated that 150- to 180-grain bullet ballistics are markedly improved by use of slower rifling twist (1:13 to 1:15 works well), compared to the factory 1:10 twist. However, with 30-inch barrels installed, ballistic difference is significant with all bullet weights.

Those looking for the ultimate long-range hunting rifle for smaller species might give this chambering a hard look. A single-shot rifle equipped with a 30-inch tube offers reasonable handling ease and, if chambered for this cartridge, would deliver huge doses of energy to a distant target with the flattest trajectory available. Accurate Arms data shows the 250-grain MatchKing generating the same muzzle energy as the .458 Winchester Magnum, when loaded to about the same pressure! So long as a good barrel is properly cleaned and not subjected to excessive rapid-fire shooting, its accuracy can remain good enough up to 2,000 rounds for long-range target shooting, and perhaps 2,500 to 3,000 rounds for big-game hunting.

Editor’s Note: This article is an excerpt of Gun Digest’s Cartridge’s Of The World.

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