338Rules,
Actually, the 6.5x54 Mannlicher-Schoenauer was the "noted slayer of Pachyderms," not the 6.5x55, due to one of W.D.M. Bell's favorite rifles being a customized M-S 1903 carbine. He called the ".256," because at that time the British often applied their names to European cartridges, the reason there's a .275 Rigby (7x57).
The 6.5x54 cartridge is even smaller than the 6.5 Creedmoor, at least in head-size and powder capacity, though it's slightly longer.
