I do not know if this has anything to do with the x53R noemclature choice, but when you expand a case to receive a larger diameter bullet, the case "shortens" {just like occurs when I neck up 8x57 cases to 9.3 caliber}. Who knows, maybe the Russian engineers measured the x54R case after an experimental trial of the 9.3 {"9mm"} round and just called it "x53R" as a result.