If you can get hold of Reloader 15 and are using bullets of 270 grains or less then the velocity advantage is quite small...down to and including virtually no gain.

300 grain bullets and especially long spitzers is where the 375 Wby (or Acklet Imp) will show a velocity advantage thant can be 200 f/s and more. Much greater than the increase in case capacity would normally indicate.

The problem with the 375 H&H and especially with 300 grain bullets and doubly so for 300 grain spitzers is that the 2208/4064 burn rate is too fast and the 2209/4350 is too slow. However, the 2209/4350/Re19 etc are close to spot on powders for the 375 Improveds.

But again, if you can get hold of Re 15 and are using lighter bullets the game changes. With bullets up to 250 grains the 4350 burn rate is a bit to slow for the 375 Improveds and the Re15 and 2208/4064 is too fast.

But overall, if you like to play about with loads etc the 375 Wby is the better deal. Also, if you are happy with standard H&H ballistics the 375 Wby will do those ballistics at lower pressure and with more powders.

From a personal point of view I would leave it as 375 H&H as I think Wby calibres should be in Wby rifles or if not Wby rifles then HS Precision, Rem 700 etc. For me, a Mauser type rifle like the CZ chambered in a Wby calibre would cause me to break out in hives.

Mike