The .338-378 Weatherby Magnum is based on the .338-378 KT, a wildcat cartridge created by Elmer Keith and R.W. "Bob" Thomson in 1966. The KT is based on the .378 Weatherby Magnum cartridge but is shorter by 1/4" and necked down to a .338 caliber bullet. This design was chosen for elk hunting, using the powders available at the time, such as Hodgdon H-4831. Due to the continued popularity of the KT, Weatherby introduced the .338-378 Weatherby Magnum in 1998 to its line of commercial ammunition, using a full-length 2.908" case. Although based on the .338-378 KT, the .338-378 Weatherby Magnum and the KT have different load data and chamber sizes and are not interchangeable.