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.
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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