hoppdoc:
Easy answer IMHO. When you hit a big animal in a vital spot other than the spine or brain with a typical NE round, there is usually no evidence of shock as such. The animal will recieve the bullet, and then most likely run, or rarely charge, depending on its demeanor at the time, and other circumstances. If the calibre is above .400 or so, the animal will probably show signs of being hit. If below .400 it probably won't, and may appear to be bolting from the sound of the shot.

With a striking velocity above maybe 2300 fps or so, there is a high likelihood that the animal will be 'shocked', and with an over-.40 bullet, it may actually drop to the shot. This will almost never happen with the .500 NE, but may happen with a stoked-up Rigby. This is non-CNS shots through the boiler-room we're talking about here.

The trade-off is that the big-slow-moving bullet is probably more certainly fatal, but not instantaneously. The faster bullet may cause instant death by shock, or at least knock the animal over, but if it doesn't, there is a higher likelihood of a long tracking job. Using premium bullets like Woodleighs will improve confidence at the higher velocity of course.

I believe the .416 Rigby with premium bullets would be the best hunting rifle for buffalo. It is lighter than a .500, easier to shoot well, and will certainly do the job. If the custard pie begins to fly however, give me a .500 double any day!

That's my take....