Ammo Brief: .338-378 Weatherby Magnum

Ammo Brief: .338-378 Weatherby Magnum

We take a quick look at a cartridge that’s kinda like .338 Lapua, the .338-378 Weatherby Magnum.

In response to special requests from close friends, Roy Weatherby chambered a few rifles for a cartridge in which the case was formed by necking down his .378 Magnum case to .338 caliber. This took place as early as the 1970s, but it wasn’t added to the Weatherby lineup of loaded cartridges and rifle chamberings until 1999.

cartridges of the world 17th ed book

The .338-378 Weatherby Magnum is sometimes confused with the earlier .338-378 Keith-Thompson wildcat, but because the K-T version is on the shortened .378 Magnum case, loading data are not interchangeable between the two.

The .338-378 Weatherby Magnum is in the same performance league as the .338 Remington Ultra Mag and .338 Lapua, and, from a practical point of view, that’s not saying a whole lot.

The smaller .340 Weatherby Magnum generates considerably less recoil, yet Weatherby rates it only 100-fps slower than the .338-378 Magnum and at about the same speed as the .338 Remington Ultra Mag., when the three cartridges are loaded with 250-grain bullets.

.338-378 Weatherby Magnum Load Data/Factory Ballistics

Bullet
(grains/type)
Powder Grains Velocity Energy Source
180 Barnes TSX RL-25 113.0 3,373 4,542 Barnes
210 Barnes TSX RL-25 108.5 3,183 4,719 Barnes
225 Barnes TSX RL-25 105.0 3,066 4,691 Barnes
250 Barnes TSX RL-25 102.5 2,906 4,683 Barnes
225 Barnes TSX FL 3,180 5,052 Weatherby B333225TTSX
250 Nosler Partition FL 3,060 5,197 Weatherby N333250PT

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


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