I don't think it has been done, and there are two rounds I would like, even as wildcats for double rifles. One is a 375H&H flanged case opened up to .416 and fire formed to the same case shape as the 416 Rem Mag. This could be reloaded with 416 Rem dies, and a 375 Flanged shell holder. Loads developed to match pressures of the 375 flanged would simply give you a better bullet without being high pressure, and much milder than the 500/416 !

The other is a flanged cased 243 Winchester, with the exact case dimentions as the rimless 243 win. so it could be loaded in regular 243 dies, with a special shell holder. This would make a fine double rifle for deer hunting in a six pound double.

Ray is right, however, almost every hole has been patched with more than one wildcat, and most of the factory rounds today, started life as somebody's wildcat, so there are still options, though few that would be absolutely NEEDED!

Wink,
If you don't mind me asking, who made that rifle for you and how much did it cost? Looks really good to me, you're lucky.