Because of the basic design of 'most' double rifles they can't handle the pressures of modern necked rounds.
The old ways are the best ways for the most part. There's no need to re-invent the wheel if it still works good.
There is a German rifle, Kreighoff IIRC that will build a 500/416 if you need a 416. I don't care for their silly 'decocker button' but that's just me.
But a 450,470 0r 500 will handle anything you want. If not there's always the 577 or 600 or 700.
Bailey Bradshaw will put a rimless round in his falling block DR. It'll cost a bunch more for it though.
