The .338-378 Weatherby Magnum was introduced around 1998 and was based on the .378 Weatherby Magnum case necked down to accept .338-inch bullets. The history of this cartridge dates back to 1966, when Elmer Keith and R.W. “Bob” Thomson (aka Thompson) developed a similar wildcat on the .378 case. Their version, however, had the shoulder set back approximately 1/8 inch (based on vintage cartridge drawings) and not the 1/4 inch described by Keith. The purpose of this was to decrease powder capacity for improved efficiency and maximize velocity with Hodgdon H-4831 powder. Cartridge drawings were completed by RCBS in February 1967, with the cartridge becoming known as the .338- 378 KT (Keith/Thomson). Both men had outfitted and guided for elk for around 30 years, and intended it to be a long-range hunting cartridge with enough authority to reliably put game down at 500 yards and beyond, which it does with ease (at least if the shooter has the skills).

With steady interest in the Keith/Thompson wildcat, Weatherby responded with rifles and factory ammunition, but its version was based on the full-length .378 Weatherby Magnum case necked down, rather than having the shoulder set back.

The two cartridges are not interchangeable and neither is load data. It should be noted that many rifles originally chambered for the .338-378 KT version have now been re-chambered to accept the SAAMI standardized .338-378 Weatherby version.

Factory loads are advertised to push a 225-grain Barnes Triple Shock X-Bullet to 3,180 fps or a Nosler 250-grain Partition to 3,060 fps from a 26-inch barrel. In a sample Weatherby Mark V Accumark rifle with 26-inch barrel, factory loads clocked 3,144 and 3,044 fps, respectively.

With such a large powder capacity, it is strongly suggested to use the Federal #215 Large Rifle Magnum primers to achieve reliable ignition. The .338-378 Weatherby Magnum thrives on slow burning powders such as Alliant Reloader 25, which gave top velocities and was accurate.