Probably the easiest and cheapest way is to rebarrel a CZ in 416 Rigby to the 500 A Square.

If you are not familiar with the 500 A Square it is basically a 460 Wby necked up to 50 calibre. Since the 460 Wby is like a 416 Rigby that has been blown out or Improved, necked up to 50 and belt added, it makes for simple conversion of existing 416 Rigby rifles.

Mike