Hi Patagon,

As far as I know the muzzle Velocity of the 06 with a 150 grain bullet was around the 2700 fps and shortly afterwards 2800 fps. So not slower than the 8x57 by any worthwhile amount.

I do not know why the US did not develop a more efficient cartridge, however the 06 did give birth to some very worth while cartridges like the 270Win, 280Rem (and the other 7mms) and 35Whelen.