I've used tin to harden up my .43" Spanish, how much depends on if you're using smokeless or black powder as I was. I made a schoolboy error in my first lot and had bullets that cast beautifully but wouldn't hit a target at 50 yards. They weren't sealing the bore and the single bullet that did hit was tumbling. I tried a 1 to 20 tin to lead ration then a 1 to 30 before settling on 1 to 25. Leading is actually caused by bullets being to hard and gas escaping past the bullet causing it to melt and not by friction. A soft bullet that "bumps" up and seals the bore is what you want.
For smokeless opinions vary to casting bullets very hard and some even say quenching them in water to further harden them. This depends on the velocity you want to shoot them at. Black powder loads are typically 1100-1330fps, in smokeless some will say a hard bullet at 2000fps works for them but at that point you probably want a gas checked bullet anyway.
I've bought tin in the past from Cain metals
Carn Metals Contact
