VonGruff,
I have examined, handled and shot the Harry Selby .416 Rigby and can assure you that it is a standard Mauser 98 action, and a military one at that. Rigby evidently made more than one rifle on the 98 action for the .416.
I would also imagine that since the standard pressure for the .416 Rigby is at most 40,000 psi there is no danger in using a standard 98--unless, of course, as you suggest a "nimrod" feels compelled to load it up to 2700 fps with a 400-grain.
By the way, Harry Selby shot out the original barrel on the .416 and sent it back to Rigby for rebarreling--when Rigby was still in England. I shot it with the 2nd Rigby barrel (Harry sold it to a guy in the U.S.) and it is still spot-on with factory ammo and the express sights.
I have also done some research into the .416 Rigby and the original factory ballistics were around 2300 fps with a 410-grain bullet, not the 2400 fps that is generally quoted today. My own .416 Rigby (a CZ 550 Magnum) "regulates" to the express sights with a handload that gets around 2300 fps--and it has worked very well on both Cape and water buffalo.
