The 240 Weatherby was added to round out the Weatherby proprietary magnum line in 1968. It differs from other 6mm's in having a belted case with somewhat greater powder capacity. It is very similar to the 240 Belted Rimless Nitro Express introduced by Holland & Holland around 1923. Thus far, it is available only in the Weatherby Mark V bolt action rifle or through custom gunsmiths. It is an excellent cartridge and will push the 100 grain 6mm bullet with about 200 fps greater muzzle velocity than the 6mm Remington and around 400 fps faster than the 243 Winchester. 

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