.........what we know: is it a real 10mm /400 and it have a paper thin neck
...........the paper thin brass case is typical for the early express rounds and the 10x60 R 400 comes from one. this is not a 450 basic case necked down.......
.........old chambers are made different than we do it now. I have casting were the chamber ends with the case mouth. no free bore, nothing, the lead bullet engrave into the rifling when loaded.....
PWM, if you drop in from time to time, can you clarify on the neck? Is there a thin neck from the 400 bore transitioning to the rifling? I ask this because I am working with a similar chamber which has a constant chamber diameter for a length of 61mm with the diameter larger than that of the bullet. Then from 61mm to 67mm there is a transition / necking down to 400 kaliber.