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The .378 Weatherby Magnum

Created in 1953, this big, powerful rifle cartridge had 43 percent more case capacity than the .375 H&H.

The .378 Weatherby Magnum

The .378 Weatherby Magnum (center) greatly exceeds .375 H&H (left) power. It derives from the case for the .416 Rigby (right). (Shooting Times photo)

After we finished editing Speer Reloading Manual #13, one reviewer took us to task because the .378 Weatherby Magnum was not included in that edition. It was neither an oversight nor a lack of trying.

Most Weatherby cartridges made the transition from proprietary to industry-standard cartridges just as Speer #13 began development. The manual team was also changing departments to become part of Development Engineering, and that took time. We were struggling to get new pressure barrels, reference ammo, and even empty cartridge cases. We had long lead times for some items and received a pressure barrel for .378 Weatherby on time. Unfortunately, we could not get enough .378 Weatherby empty cases to do the tests before our press date.

We had more time and better support for Speer Reloading Manual #14, including techs shooting in two separate labs, not one as before. There was also time to get enough cases laid in, so my final manual had .378 Weatherby data.

The .378 Weatherby Magnum dates to 1953, when Roy Weatherby sought a significant power improvement over the .375 Holland & Holland and the .375 Weatherby. The basic H&H case was the fattest, longest case used for American rifle cartridges at the time, but Weatherby needed more.


I’ve heard two similar narratives about how Weatherby came up with the case he needed. One says the .416 Rigby was the inspiration; another says he slapped a belt on the .416 case—after all, in the 1950s anything called “magnum” had to wear a belt. The Weatherby version was roughly a 43 percent volume increase over the H&H.


It’s hard to say which is correct because the two cartridge drawings don’t use the same reference dimensions. The .416 Rigby follows European CIP practices, and the .378 Weatherby uses SAAMI conventions. Still, measuring unfired cases at a point 0.275 inch ahead of the base shows the Rigby body diameter is only 0.003 to 0.004 inch larger than the Weatherby.

Did Weatherby get what he wanted? I’d say so. Weatherby numbers from 1964 and today have the .378 moving a 270-grain bullet to 3,180 fps and 6,050 ft-lbs of energy. It drives a 300-grain bullet to 2,925 fps and 5,700 ft-lbs. Weatherby’s velocities are from 26-inch barrels. When I compare those 1964 values to numbers for the .375 H&H, a one-third greater muzzle energy is a stunning increase, even considering the shorter 24-inch H&H test barrel.

With any older high-capacity cartridge, a handloading question arises. Can newer propellants increase performance? It has happened with other cartridges that were developed before sufficiently slow propellants were available that could take advantage of the case capacity.

Speer Handloading Manual Number 15 data for .378 Weatherby has three newer propellants to which I did not have access when Speer #14 was still in development (2004–2007). Because the Trophy Bonded bullets were not included in Speer #15, the only bullet common to both data sets is Speer’s very successful 285-grain Grand Slam softpoint. In 2004 the highest average velocity we achieved from our 24-inch Krieger test barrel was 2,996 fps, fueled by 115 grains of IMR 7828.




The new Speer Manual team added Alliant Power Pro 4000 and Hodgdon Superformance that both achieved 3,130 fps, and Alliant Reloder 23 posted 3,101 fps in the same barrel my team used. That brought 285-grain handloads closer to 270-grain factory ammo. Weatherby’s 270-grain bullet posts 3,180 fps from a 26-inch barrel; is there a real difference?

A very crude assumption of losing 25 to 40 fps per inch of barrel suggests the factory ammo with a 15-grain lighter bullet would leave a 24-inch barrel at between 3,100 and 3,130 fps. That’s essentially a tie in velocity, although the heavier Grand Slam adds about 5 percent to the muzzle energy. To my ballistician’s eye, Roy Weatherby came up with a case that was nicely optimized for propellants from 1953 as well as offerings from today.

Remember that one pound of propellant is 7,000 grains. When loading 100 grains per case, buy two pounds if you plan to load more than 70 cartridges!


The bullet has to match both the game and the velocity capability of the .378 Weatherby. Things you shoot with a .378 are big and dangerous, so bone-busting penetration is important. For Speer #14 my team opted not to show the popular Speer 235-grain semi-spitzer SP for two reasons. At .378 velocities, a game bullet that is perfect in the .375 H&H can be overdriven, reducing retained weight and penetration.

The other reason was ballistic consistency. Light bullets in huge cases often produce wide swings in pressure and velocity. We found the 235-grain bullet was simply too light for the .378’s volume, although the tough 250-grain Trophy Bonded Bear Claw SP made the cut. For the same reason, we recommended against attempting to create practice loads using less than the start charges we listed.

As I researched this, I saw the .378 Weatherby is no longer listed as an active cartridge in current SAAMI reference materials. Still, Weatherby sells rifles and ammo today. A perennial problem with .378 Weatherby popularity was an odd legal one. Some African countries made you use a .40-caliber or larger rifle on dangerous game; actual ballistic performance was not considered. The fact that the .378 could churn up 1,000 ft-lbs greater energy than the .458 Winchester Magnum made no impression on the bureaucrats.

Recoil comes up in .378 Weatherby discussions. A long-standing vision issue has precluded me from bench-testing the “boomers” for quite a while, but I had a circle of acquaintances when I worked at Speer who phoned in their adventures—and misadventures—with testing African-class rifles. They ranked the .378 high on their “ouch-o-meters.”

A footnote: Writer Jack O’Connor acquired a custom Apex-Enfield .416 Rigby rifle in 1953 and found that factory ammo was nearly impossible to get. He asked Roy Weatherby for .378 Weatherby empty cases, then turned off the belts on a lathe. Adding normal case-forming procedures, he could handload for his Rigby. That’s how close the two cases are.

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