338-378 KT (338-378 Weatherby)

Historical Notes:

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.

General Comments:

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. Should loading data become available for some of the newer powders between H4831 to H870 in burning rates this cartridge could soon gain new popularity. The only data we could find is for H4831 which is decidedly too fast burning for this number. Still, velocities exceed what the 340 Weatherby Magnum can offer by about 10% when loaded to similar pressures. Properly loaded with a somewhat slower burning powder this chambering should be able to gain about 100 fps more velocity advantage. When combined with the more aerodynamic hunting bullets available, this number can easily deliver substantially more energy at one quarter of a mile than the 30-06 produces at the muzzle.

Source: Cartridges of the World

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