In 1920 H&H; brought out their .300 Magnum cartridge, based on the 375 H&H; Magnum case necked down to accept .308 inch bullets, which is called the .300 Belted Rimless Magnum or H&H; Super .30 in Britain. Its case has been the basis for most of the magnum cartridges designed ever since. The traditional British factory load drove a 180 grain bullet at a muzzle velocity (MV) of 2750 fps with muzzle energy (ME) of 3020 ft. lbs. 

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