Hello there,

I too have been bitten by the big H&H cartridge bug and have had a .465 H&H reamer made for my project.
My only suggestion is to resurch the project thoroughly before rushing in. I am finding it hard to get a barrel and brass for the 465.
Brass in your case shouldn't be a problem a mate of mine fire forms factory 375 loaded ammo out to 458 lott so this should work just as well for the 400.

Loading dies are another problem though if you have a reamer dies can be made.

To make things easier I am contemplating re-sizing the neck of my 465 reamer to 458 witch will give an endless supply of barrels and projectiles.

Good luck with your project mate.