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Nobody would have been hand loading this cartridge back when it was created.
Factory ammo was the only source.
Kynoch 400-350 Rigby ammo (similar to 400-360 WR) from that era measures only .038" difference between shoulder diameter and outside neck diameter!
That means the 'step up' from the neck to the shoulder is only .019"

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This isn't quite correct. To avoid confusion it probably needs clarification although the info is already provided in the thread.

The 400/350 Rigby case is similar to all the other 400/360 variants (but not the Westley Richards version).
They all have a similar length at 2.75", similar shoulder diameter at .415" and different neck sizes depending on the bullet diameter used in each case. They are all rimmed cases and head space off the rim so the small .038" step at the shoulder of the Rigby case or the .028" step at the shoulder of the Purdy case is irrelevant. This is machined into the chamber. It has nothing to do with hand loading.

The 400/360 Westley Richards case is a similar length at 2.75" but the shoulder dia is larger at .437" and the neck dia is .375" giving a step at the shoulder of .062". This case is also rimmed and head spaces off the rim so the larger step at the shoulder is irrelevant. The rim appears to have been turned off this case to create the "Rimless" variant which is the topic of this thread. Now the larger .062" step at the shoulder becomes relevant as the cartridge has to head space from this shoulder. There are plenty of rimless cartridges which head space off a .060" shoulder so it shouldn't be a problem. Maybe Westley Richards had a rimless version in mind when they designed their 400/360 case different to all the others. I don't know.

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