Now if I could just find the 300 RUM brass....
Well, unknown friend, I don't have any better idea than you about that.
An ignorant ? Can 300 RUM be formed from any other known cartridge? IIRC (if I recall correctly) that is a beltless case so.... I'm not up on some possible cases that you could make 300 RUM out of.
I'd help IF I COULD so good luck!!