Roy Weatherby created the .375 Weatherby Magnum in 1944 but not long afterward decided that it was still underpowered. He set out to create an ultra-powered .375 cal. and used the 416 Rigby case as parent design, but with a belt added and combined with the double radius shoulder, gave it a true ‘Weatherby Magnum’ appearance. In the beginning he experienced problems in that current primers available at the time did not have the ‘oomph’ to ignite powder charges north of 100 grains. Federal created the #215 Magnum primer specifically for the Weatherby Magnums (see description on the box copy). His new design was named the .378 Weatherby Magnum and was introduced to the public as a Weatherby proprietary cartridge in 1953.