Ammo Brief: .338-378 Weatherby Magnum

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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.

General Comments

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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