I have a couple of pm's on this subject and a few more about another 40 so I thought I'd answer some of those questions here and perhaps other could benefit from my meager experiance on the subject, or at least ease their curiosity a bit.
The original 400 Whelen was developed by a group of experienced and knowledgable men back in the 1920's. Townsend Whelen, James V. Howe and barrel maker Adolph O. Nieder. I believe that Fred Adolph, who developed the 30 Adolph express, later called the 30 Newton, was also involved in this project. The caliber was a brain child of Whelen, the rifles made by Howe and the barrels were made and installed by Nieder. There is some evidence that Whelen and Adolph corresponded on the 38 Whelen, which died away after Winchester discontinued the 275 grain 38-72 bullet, and likely corresponded on the 400 as well. Of course the rifle makers of Griffin&Howe and Hoffman were involved in making many rifles for Whelen designed cartridges.
I have always liked the 40 caliber rifles and the 400 Whelen peaked my interest many years ago after reading about in in one of Elmer keith's books. My first rifle in the caliber was a miserable failure because the smith who made it used a reamer that fllowed the tapered case dimensions of the standard 30-06 and there wasn't enough shoulder to headspace the cartridge. My second rifle was made with a reamer redesigned and made by a machinist friend of mine with the shoulder diameter expanded to leave about .016" of taper in the 30-06 case. This one worked well and I used it for a few seasons on elk before it was traded to another friend.
The base of the 30-06 case is .473" in diameter the shoulder is .441" The diameter of bullets for the 400 Whelen is .411" That leaves .030" brass wall thickness and shoulder to headspace on. Not enough. The brass wall thickness will be about .024.028" which basically leaves the case like old no-shoulders the snake. My "improved" version was expanded at the shoulder to about .457" and the shoulder angle was changed to 25 degrees from the 17.5 degrees of the original. I later read an article in a magazine about it that indicated the original Whelen was made from cylindrical brass and necked down leaving the shoulder at minimum body taper and a diameter of .458" with original shoulder angle. I have recently found this information on the Z-Hat webb site about the 400 Whelen and I think it also applies to the 411 Hawk caliber that Fred Zeglin makes as well.
To apply this to the practical side of life what this means if you want to make a 400 Whelen make sure the reamer will cut a chamber that will allow you to use expanded brass or the cylindrical brass neck appropriately.
I no longer have a 400 Whelen. I would like to have another so marked as "400 Whelen". I don't want a 411 Hawk, not because it isn't a good working caliber, it is, but because it wouldn't say "Whelen" on it. I think I'd like to build a 400 Whelen on a Springfield action, just the way Whelen did it.
I have in the mean time, developed a very good working caliber, based on the 376 Steyr case. This case is a little fatter in body than the '06 case and will leave a more substantial shoulder. I further expanded it to .486" at the shoulder and changed it's shoulder angle to 25 degrees, to not only give a good headspace datum, but it will headspace correctly even before fire forming. Not that fire forming is a task, but all brass fits better when it is fired in ones own rifle and many factory rounds fail to headspace with a crisp, positive lockup. This 376 has slightly more powder capacity and improved ballistics. It still has .400" of neck, it has a positive headspace datum, and it will fit and function in a standard Mauser action and magazine, even with a 400 grain bullet. It's purpose in life is to give 450/400 Nitro Express ballistics from a standard length bolt action, this being 400 grains at 2150 fps. It also will exceed the 405 Winchester ballistics, as does the original 400 Whelen, with the 300 grain bullets so readily available today. The expected mark with the 300 grain bullets is 2600 fps and 2250 fps with the 400 grain bullets. This is serious lion medicine and I'm sure Theodore would agree. I wonder what Mr. Whelen would think of this one.