The 338-378 Keith-Thompson was developed to deliver big heavy bullets to distant big game animals with a flat trajectory. Elmer Keith always advocated the 33 bore was minimum for elk hunting. The advent of the 378 Weatherby case gave him ready access to a larger capacity case that would allow equal weight bullets to be launched several hundred feet faster than was safely possible with existing .338 inch magnum's. Evidently this was an opportunity Elmer could not pass up. The 338-378 KT holds more than 120 grains of powder, compared to about 90 grains for the 340 Weatherby (then the largest .338 inch case). With the advent of new slower burning powders, suitable for handloading in this cartridge, the 338-378 KT should gain added support among the "Bigger is Better" genre of hunters. 

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