Sounds good, however, depending on the penetration NEEDED, a 10:1 through to perhaps even 40:1 lead/tin would likely work well. Slightly softer alloys will not penetrate as well, but will obturate in the bore better with black powder loads and thus may shoot the best of all - for those, perhaps 30:1 to 40:1, or a 50:50 WW/Pure Lead mix that shot best in one of my BP rifles I should not have traded off.

Back in the day, mercury was used as a hardening agent and later on, tin was used. Just when the switch-over to tin as a hardening agent, I do not know.

How well mercury hardened the lead, I do not know either - however Forsyth mentioned this in his 1863 book.