The .340 Magnum uses .338" bullets, same as the .338 Magnum, and it is Roy's fifth most popular cartridge; the .300, .270, 7mm, and .257 Magnums are ahead in popularity by a considerable margin. The Weatherby Mark V is the only factory rifle available in .340 Magnum, but a number of custom rifles are built in this caliber around the Remington Model 700 and Winchester Model 70 actions. When loaded with the right bullet, the .340 Weatherby Magnum is suitable for hunting any of the worlds big game. This includes elephant and cape buffalo (where the caliber is legal) when the .340 is loaded with bullets of full metal jacket construction. For all thin skinned game the handloader can do a lot worse than load H4831behind a Nosler250 grain Partition. An excellent long range elk and moose load for the .340 is the Nosler210 grain Partition or the Sierra250 grain spitzer pushed to maximum velocity by H4831, H450, IMR-4831, or IMR-7828. 

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