Hungry Horse, back in 2008, when the writing was on the wall so to speak, I started experimenting with non-lead roundball options in my slow-twist muzzleloaders.
The best option I found that was home cast, was 85% Bismuth, 15% tin. You have to use a mould that is a tad smaller than for lead because bismuth expands when it cools, but the result is a dense, "relatively" soft projectile, that will not harm your barrel. I got hunting accuracy-- meaning I could keep em on a pie plate. They penetrated well but had virtually zero expansion. Sometimes they would crack into two pieces if they hit something really hard, but they don't bounce.
The other option I found that is NOT home cast, is ITX non-lead roundball. Its pretty hard stuff but it is softer than barrel steel and will not harm your barrel. But just to be safe, I always use a leather chamois greased patch to help protect my bore. It has inherent issues-- you cannot pull it if you dryball, it will bounce off hard surfaces, but it has superior penetration.
ITX non-lead shot is a great option for muzzleloaders. It patterns great and will not harm the bore. You can crush a pellet easily with a pair of pliers, They have been known to create a better pattern than lead. Good stuff. I will be using it this year on squirrels and turkey because the law kicked in this year--very sad to say.
I have a goodly supply of ITX roundball for my 50 cal rifle, and ITX shot for my smoothbore, because I was a tester/prostaff for Tombob Outdoors. I only use it for hunting---its spendy stuff.
K