25-45 Sharps and 6mm/.204 are wildcats, but there's a bigger and maybe better CIP-registered cartridge known as the 6x51 ATZL.
Few know of it. This is another straight neckup, but this one is of the 5.6x50 RWS Mag. Available as a calibre option in Blasers, and as a standard chambering in a micro-action from one or two custom rifle- makers. CIP drawings are copyright, so the link is here:-.