Edit: 404J also has a much longer case than 400 H&H. The latter must have less case volume but shooting out same weight 400 gr bullets faster than 404. Interesting.
Regarding your prior post, in checking the case capacities of the 400 H&H, 416 Rem and 404 Jeffery, this information is from Pierre van der Walt,
African Dangerous Game Cartridges, copyright 2012:
400 H&H 99.3 grains (CIP)
416 Rem 107 grains (SAAMI)
404 Jeffery 113.3 grains (CIP)
After reading Mr. van der Walts chapter on the development of the 400 H&H and the 465 H&H, it reminds me of the marketing term in my industry (food product development) of the creation of
Signature Products for the storied Holland and Holland name.
Mr. Russell Wilkins of H&H wanted to create a .40 caliber, as well as a .465 caliber, marketed under their own brand. A nice option for their buyers of a bespoke rifle.
The design of the 400 H&H is interesting, in that it has enough case taper, 8-degree shoulder angle and long neck similar to the 375 H&H. Sure, the case taper from the base to the neck is less but is enough.
It's interesting that Mr. van der Walts' son chose it for his work as an apprentice PH.
Just my .02 cents.