The .50 BMG has been necked-down to .338. The late, great Skip Talbot did it. The ".338-.50 Talbot" is listed in the latest edition of Cartridges of the World. Muzzle velocity is 3700 FPS with a 250 grain bullet. Barrel life is rather short, due to throat erosion. Imagine that.
The .378 Weatherby has been necked-up to .458. In 1946, IIRC. Can you say, ".460 Weatherby?"
One of the most interesting cartridges I've seen (that actually shoots) is the "Fat Mac .50" Developed by the late, great Gale McMillan. It's the 20mm Vulcan cartridge shortened and necked-down to .510. Gale described it as "sort of a .50 PPC" It will shoot a 750 grain bullet at 3500 FPS. That's over 20,000 foot-pounds of muzzle energy.
