Cast bullets that have a decent lube groove, not the ones that have tumble lube cross hatching or shallow lube grooves. Slug your barrels and size your bullets accordingly. At least .001 larger than the bore diameter. Use a swage type lube/ sizer to do this. After firing check and see if you have a lube star on the muzzle of your piece. That’s to say a star shaped splatter formed by bullet lube on exit of the bullet from the barrel. That will tell you that you have enough lube on the bullet. If the bullet doesn’t have enough lube it can lead the barrel. While this is important to check with a pistol it’s doubly so with a rifle shooting cast bullets.
I’ve shot thousands of rounds of .44 Mag, .357 Mag, 30-30, 375 Winchester, 45/70, etc. with cast bullets at maximum loads and then some. Using straight wheel weight, range scrap and a bunch of other alloys properly sized and lubed I’ve never had any leading.
Those who beat their guns into plows, will plow for those who don't.-Thomas Jefferson