I agree with the link 4seventy supplied. Basically, when many of the gunhouses tried to make the 400 Whelen, they kept the '06 body taper resulting in an insufficient shoulder. Also, it was doubtful that many paid enough attention to chambers.

In working with push feed rifles chambered for the various "improved" 400 Whelen variants, I haven't heard of any new headspace problems as long as the shoulder is kept at the .445" as determined by the good Colonel.