brilliant snider marrakai, do you ever shot pics with this? suspects its a .577

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450 No. 1 is indeed a problem

454 casull rim diameter is under the base diameter so it dont work

there is no case you dont have to change rim diameter and rim thickness on the lathe

11,7x45 R danish remington comes very close but its rare and berdan primed

11x58R spanish remington is a good match but expensive and sometimes hard to get

8x50R Lebel is a little bit over on the base but the case is very conical and I am sure its possible to reduce the little bit oversized base diameter on the lathe without weaking it for a blackpowder load. reduce rim diameter and thickness also, aneal case mouth and blowing out with a fire form load.
the brass is common, cheap and available from PRVI