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Notes from the Lab: .338-378 Weatherby Magnum (using Sierra bullets) The .338-378 Weatherby Magnum was introduced by Weatherby around 1998. The cartridge is 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 developed a similar wildcat based on the .378 case. Their version, however, had the shoulder set back approximately 1/8 inch (based on vintage cartridge drawings) and not the ? 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 new wildcat becoming known as the .338 -378 KT (Keith/Thomson) intended as a long-range hunting cartridge with enough authority to reliably put game down at 500 yards and beyond. With steady interest in the Keith/Thomson wildcat, Weatherby responded with rifles and factory ammunition; however, its version was based on the full-length .378 case necked down, rather than having the shoulder set back. Regardless, 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 wildcat have now been re-chambered to accept the SAAMI standardized .338-378 Weatherby version. Factory loads are advertised to push a Barnes 225-grain 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 Mark V Accumark rifle with 26-inch barrel, factory loads clocked 3,144 and 3,044 fps, respectively. With the large powder capacity associated with this cartridge and to achieve correct powder ignition under all conditions and temperatures, it is strongly suggested to use large rifle magnum primers such as the CCI 250, Remington 9 1/2M or Federal 215, with the latter used in the accompanying data. ................
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