As has been stated, the Rigby will give greater recoil with typical powder charges (ie 100+ gr of 7828) vs the 416 Rem using less of the faster powder...even when the MV is the same (the powder counts at 1.5x on the ejecta mass calc).
But when it is time to crank it up, the Rigby gives more options at safe pressure than does the Remington.
I have some 400gr loads with IMR 4064 that I am looking forward to chronographing. I hope to learn how accurate a 404J duplicate load I can make...with <80gr charges...to show if the Rigby versatility will include producing 404J ballistics and recoil impulse. I have the loads with and without filler, to see if there is a measurable difference in consistency and average velocity.
I think Boddington summed it up nicely in the old Nosler loading manual intro piece to the 416 Rigby.
He said (I paraphrase) the Rigby isn't better than the other 416 cartridges...but if tradition matters to you, get the Rigby.
