I understand the hardened bullets were with tin and/or mercury. I tried the mercury hardening back in the late 70's, but wasn't impressed with the results. Tin and antimony were much better for this.

The advent of the ctg. rifle allowed hardened bullets as I understand the situation. Hardened bullets could not be used in muzzleloaders due to their lack of accuracy. Only pure lead bullets could be used & with their excessive expansion, their penetration was poor. The advent of ctg. rifles in the BP era allowed the use of hardened bullets. Only then did conical bullets surpass the hardened round ball in penetration on dangerous game. We are talking about bore rifles now.
Samuel Baker was a staunch supporter of hardened 'balls' for the muzzleloading bore rifles use on dangerous game.