Its a true rebated rim cartridge, do you have any problems with cartridge feet from a Mauser magazin?
The bolt slide over the case head when the feet isnt work very good for the cartridge. This problem dont stand in a Westley Richards .425 magazin but i think your cartridge is to short to make the problems we see on longer rebated rim cartridges.
Your new wildcat, he- do you have an official name now, is very similar to the .45 Blaser.

577 Robert I hope I get all from the 9mm AE to the 55x175 RB, you will see it in the DWJ some day