Brother has a Number 1 in in 416 Rigby that I've done all the reloading for. Nothing really specifically different about that caliber over any other that I've noticed. What's your question?
I have had 6 416s down to just 4 now . They are better with jackets bullets .The Barnes 350 grain is awesome .I also shoot speer 350 grain they shoot awesome .
Haven't loaded cast in the 416, but have shot plenty of it in other large volume cases and I'll second the XMP5744 nod. Have used 5744 in the Rigby with jacketed 350's and very pleasant.