I shoot magnum level .41 Mag in the 1300 FPS range using air cooled wheel weight alloy. If you are getting lead in the barrel it is more times than not from undersized bullets. Making the alloy harder will not stop the problem, and many times will make it even worse.
If your bore is clean by the forcing cone, and there is lead near the crown then it is a lube issue. (Note be sure it is lead, and not carbon from lube run a solvent soaked patch, then a few clean dry ones. If it is lead it will still be there. If was just burned lube it will be gone.)
If you have lead in the barrel at the forcing cone that follows the rifling then your bullets are undersized, and powder gas is cutting around them. It tends to be worse closer to the chamber.
If it looks like you can not see the rifling, and the barrel appears to be plated with lead. Your powder is too hot. You should either gas check your bullets, or use another powder.
Harder bullets shold be even more oversized than thier softer counterparts. The softer bullets deform at the base to create a better seal in the barrel. If they are harder then the sealing effect is somewhat negated.
There are some here that can explain it a whole lot better than me. I am just a janotor that likes to cast, shoot and reload when I am not on the net or drinking an ice cold bottle of suds.