The term magnum is of Latin origin, and is traditionally used to refer to an oversize bottle of wine or champagne. It has come to mean an oversized cartridge, generally with a belt at the head of the case. The first magnum cartridge to be introduced was the .375 H&H Magnum, originally known as the "Nitro Express." It was followed by calibers such as .300, .244, and .275, all based on the same design. In the US, Roy Weatherby modified the H&H cases to develop his Weatherby Magnum family of cartridges. Many were designed to be used with a standard .30-06 rifle action. Roy Weatherby's developments set the pattern for subsequent magnum cartridges This cartridge was first introduced in 1968 for the Weatherby Mark V rifle, Weatherby was competing at the time with the .244 Holland & Holland Magnum cartridge. This was the last Weatherby cartridge in the Weatherby line of belted magnums.
The .240 Weatherby Magnum is a special cartridge used only in Weatherby rifles. This distinct Weatherby case is exclusive and it is not based on any of the other Weatherby rounds. It has the same diameter and comparable length to the .30-06. It does have the established Weatherby rounded double shoulder and a belted case with the length of 2.496 inches which is considerably shorter than the 2.8 inch case of the .244 H&H Magnum.
While favored by some varmint hunters, the .240 Weatherby Magnum is unpopular with some reloaders because the case cannot be formed out of any other brass. You either pay the money for regular Weatherby ammunition or purchase Weatherby cases which usually more costly than those most other cartridges. The .240 WM is still the fastest commercially created 6 mm cartridges. There are a few wildcat 6mm with faster velocities, but the .240 Weatherby betters the 6 mm Remington and the newer .243 Winchester Super Short Magnum by about a bunch feet per second. With the heavier bullets the .240 Weatherby Magnum is a first-class deer hunting round and we have used it for antelope with great results. We have found a long barrel results in its peak performance with the heavier bullets.
Some other sources of information about the .240 Weatherby Magnum are:
A good source of Weatherby Ammunition for the 240 Weatherby Magnum 85 Grain Barnes Triple-Shock X-Bullet Lead-Free Box of 20 is http://www.midwayusa.com