I knew a french sportsman (not a PH)now deceased, who has shot over 119 forest elephant in both Cameroun and Gabon Over 30 plus years.
He claimed that With the .378 wby. one can apparently penetrate completely through the thickest part of an elephants skull and that no other cartridge caliber can do that in his experience..
He compared this to the .460 which he also loved but said typically the .460 did not go right through at the same location on the skull..
He said he was rarely able to recover bullets from the .378 but had plenty of .460 wby souvenirs.
I have no reason to doubt these clams.
I heave heard and read over the years, other people claiming the same things, citing the .378 wby as the penetration King..
One thing I can say about the .378 wby, is that because of that wby style stock, It kicks like all hell !
Chamber a Ruger no.1 in .378, Magnaport it and It's much more tolarable.

Tom