Efrem, your first problem is DEFINING hard cast. Is it WW, HTWW,Linotype or some thing else. Is it HARDER than 15 BHN or 28 BHN.?
Are you using a rifle or hand gun? Or is this a revolver load. Do you worry about leading or just accuracy? Most of MY loads are self cast of an alloy that resembles Lyman #2 at around 15BHN. I worry MORE about the accuracy of the load than any thing else. IF I can put the bullet where it NEEDS to go, I have a dead animal to take care of !