Both the .404 and the 12.7x70 Schueler aka .500 Jeffery were designed by companies who could not get Mauser Magnum actions at that time. Remember, up to WW" Mauser frowned upon the idea of opening standard length actions for longer cartridges. Holland &Holland used magnum length actions exclusively on their pre-WW2 .375 Magnum rifles, they only used, like Rigby, opened up standard length actions in the decades after WW2 when magnum length actions were not available any more. Today, German proofhouses carefully scrutinize opened up actions set up for the .375 H&H, .404. They will probably not allow an action opened up for the .416 Rigby. According to a Mauser table in Jon Speed's book the .404 and the .500 needed a magazine box 82mm long, the .375H&H 85mm, while the .416 Rigby needs 87.5mm. As the opened up action for the .375 is right at the absolute borderline, opening up an additional .1" is past that borderline. The difference in Magazine length from .404 and .500 to .416 Rigby is 5.5mm, exactly the length difference from the standard to the magnum length.