Ripp a 416 Rem Ultra Mag is very close to necking a 404 case down to 416. The Weatherby and Rigby (loaded up) will out perform it but why bother. Same neck the 300Ultra mag to 404 and you pretty much have a 404Jeffery.

I am thinking about necking a 338RUM up to 358. I think if I was going down the 416 route again I would go with the 416RUM. Slightly cheaper brass and easier to get an action to fit than a Rigby, then again maybe not.