The .378 Weatherby Magnum was designed by Roy Weatherby in 1953. It was an original belted magnum design with no parent case, inspired by the .416 Rigby and headspacing of the belted .375 H&H Magnum. The 215 magnum rifle primer was developed by Federal specifically for this round. The cartridge can hold upwards of 7.13 g (120 gr) of powder. The 378 Weatherby Magnum cartridge also has the double radius shoulder design found on the first and smaller proprietary line of Weatherby magnum cartridges.
The motivation behind the development of the .378 came from the disappointing performance gains of the improved,.375 Weatherby over its parent case, the .375 H&H Magnum. Roy Weatherby in 1953 killed an African elephant with one shot while on safari. However, in using this event as a marketing tool, it was revealed some African countries have a minimum 10.16 mm (.40 caliber) bullet size for hunting dangerous game. Weatherby responded by necking up the .378 to 11.63 mm (.458 caliber) and called the new cartridge the .460 Weatherby Magnum, which was introduced in 1958.
It is considered a Safari grade cartridge. The .378 Weatherby Magnum is appropriate for taking all African game animals from large African antelopes, Nile crocodile, hippopotamus, to the Big Five game. Some hunters on the North American continent employ the .378 for American elk, Brown bears, and polar bears.
The .378 Weatherby will generate considerable free recoil, an average of 104 J (77 ft·lbf) from a 4.1 kg (9 lb) rifle. This compares to 27 J (20 ft·lbf) from a rifle chambered for .30-06 Springfield. But is only 10% more than the recoil of the .375 H&H Magnum while having 22% more energy. It has less recoil than almost all cartridges of its energy. While the recoil is stiff, exaggerations have likely done a lot to reduce the popularity of this caliber.
The .378 has been responsible for numerous wildcat cartridges, being necked-down as the .22 Eargesplitten Loudenboomer and necked-up as the .500 A-Square. Some of the .378 Weatherby Magnum wildcat cartridges are shortened versions, like the .30-378 Arch (7.62 mm) and the .460 Short A-Square (11.63 mm). Some .378-based wildcats have gone on to be part of the Weatherby line: .30-378, .338-378, .416 and .460.