One of the first cartridges by Roy Weatherby was the .375 Weatherby Magnum, an improved version of the .375 H&H; Magnum. Weatherby's .375 was a good cartridge but it beat Holland's .375 by only about 100 fps. In 1953 Roy dropped the .375 and introduced the .378 Magnum. Despite nomenclature, bullet diameter remained the same. But the same cannot be said about velocity; the .378 pushes a 300 grain bullet along at close to 3000 fps. The .378 Weatherby Magnum was inspired by the .416 Rigbycase which is quite similar in dimensions but without a belt. Roy added the belt because all his other cartridges had one. Among American designed big game cartridges, only the .416 and .460 Weatherby Magnums, cartridges identical to the .378 except for caliber, burn more powder. The .378 Weatherby Magnum is an extremely powerful cartridge. When loaded with a Nosler300 grain Partition bullet, it is suitable for use on any big game animal in the world, but the shooter has to pay a price for such performance; the .378's recoil is far too vicious for all but the most seasoned shooters. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.