Con,

Actually the .400 H&H isn't just a .375 opened up to .411". Shoulder is more backwards and sides are a bit more parallel than in .375. Actually it's the shoulder that is more "old school" than in .375 and it is longer. Anyway, it looks a bit like .404 and 450/400 if you look at the side profile of the cartridge.

Basically the neck is as long as the seated shank of the bullet and shoulder starts from there. I think this is for the reason that there is no excessive space that a seated bullet will take away and therefore it can operate with a bit less powder. Pressures aren't too high and not too low either. Too low pressures in too big case often result to uneven ignition of the powder and unpredictable chamber pressures which is a thing that bothers me as a reloader in some of the old cartridges and more modern cartridges that are not driven to "healthy pressures". Anyway, I think that in some cartridges the pressures are easier to keep in control than in others.