"....Ruger makes a 416 Rigby, but looking at the ballistics of the Rigby and Rem Mag the data is contradictiry...."
Go to Barnes #3 and you'll have the straight scoop; the 416 Rigby can be loaded to near 416 Weatherby levels; in fact hold a Rigby round and a 416 or 460 Weatherby round in your hand and you'll see where ole Roy got the idea.
Ammo manufacturers? They won't go past the old 1911 cordite specs.
Handloaders? Oh yeah, the Barnes data let's you really stretch her legs, the old girl has a lot of life in her.