You said harder lead would lead BP firearms if used. I always understood the biggest reason for softer lead was to let it (the bullet) obturate, and saving stress on loading rams, but I didn't think harder lead would leaf up the barell. I don't have issues with leading on any of my smokeless pistols using WW, so why would it lead in BP.
Your really need pure lead - 99-100%. Soft enough that you can scratch it with your fingernail, easily.