Depends on the game and bore size, I would say. Hardened round balls (with tin or mercury) were used in Africa and India for shooting
dangerous game. The bullets of the day, mostly hollow based conical 'balls' needed to be dead soft to 'take' the rifling and for that reason,
conicals did not become particularly popular for shooting dangerous game until the advent of the breech loader where hardened conicals could
be used.
So- what does this all mean? - for NA game, a dead soft round ball, appropriate to the game size, is all that is needed. With larger calibres
such and Bob's 10 bore (.77) or my buddy's rifle 11 bore at .75cal. having all the diameter necessary for the most grievous wounds, hardened or
soft balls can be used.
Yes, the hard ones will exit. My buddy had exits on every moose using his .75 with hardened balls until he dropped his load down to 120gr. 2F.
With 150gr. and over(200gr.) he had exits from every hit, no matter the angle. He used hard lead (WW) as he didn't have any pure lead. It is
possible the 150gr. load (1,350fps) would not have exited if a pure lead ball was used.
I would suggest pure lead for .54's and .58's for game up to and including moose/elk. Just shoot them tight in behind the front leg and you are
good to go. Stay away from the large bones. Those will stop a pure lead ball, preventing it from entering the chest cavity, even from my .69, however
a WW ball smashes right on through.