Jamie, I do not, nor ever have, owned a 416, but have had many clients & freinds use them on both buffalo and wild oxen.
Yes, I understand that with modern powders and careful handloading velocities capable from the Rigby cartridge today exceed those gained in the standard, original factory offering.
Please read below before proceeding.
I can tell you this from my observations.
When used with standard loaded factory ammunition the 416 Rigby is an excellent performer on all bovines, so long as the minimum standards are maintained (good shot placement, good performing projectile).
When used with carefully constructed handloads, utilising premium grade projectiles and loaded within the original Rigby load limits,or even slightly below, it performs very, very well indeed.
When handloaded by zealous velocity seekers beyond the original ballistics, utilising bullets that were originally designed to perform at the standard Rigby benchmark of 2400f.p.s. or less, and coupled with typical big game hunting scenarios of close range shots, thereby resulting in bullet impact at extreme velocity, the performance of this cartridge (due mainly to bullet failure) becomes FAR less effective, unpredictable and bordering on haphazard.
I have faced more determined charges from Buffalo AND wild Oxen from animals being hunted with excessively handloaded 416 Rigbys than all other cartridges combined !
If it is a desire of yours to obtain maximum velocity from this cartridge I stress you would be well advised to consider being limited to using the very best premium grade, strongly constructed projectiles available on the market (preferably lead-free), as I have seen very few (none) lead core .416 bullets that can consistantly perform beyond that 2400 f.p.s barrier.
My view has been created through my own personal exposure to the use of this cartridge by others on buffalo sized game and ofcourse is not absolute nor infinite.
Others views may differ.
All I do know is that I have seen over-loaded .416 Rigby's, in terms of velocity acheived, result in some terrible bullet failures and dramatic final moments of hunts.
As one good, knowledgable, freind of mine allways says,
"velocity is not ALLWAYS your freind !".