great writeup on this cartridge in a recent rifleshooter mag (may/june 08).
Jon R Sundra got the 416 JRS going using published 416 Rem mag starting loads and working up, as he found case capacity to the base of the neck to be the same in the two rounds.
One load he used was a 400 gr Hornady Soft Point , 75 grs of RL15 and was getting 2405 fps. The other load he published was a 350 gr barnes TripleShock on top of 75.5 grs RL15 at 2,570
Oh ####, i just re-read the post title and your looking for a wildcat off of the 416 Rigby. Oh well, theres numbers for a 416 Ruger above, lol.