They use commercial "hardball" alloy. That is 92% lead, 2% tin, 6% antimony. Linotype is 84% lead, 4% tin, 12% antimony. (which is why it makes such light bullets)
That hardball alloy comes out around 21 BHN as cast, but if you oven heat treat it. (as close as you can get to slumping, or about 475 deg. for an hour and VERY QUICKLY quench in water they will go to 30 BHN)
Lino will not heat treat, something about it being a eutectic and for some reason won't harden any more than it already is. A real metals guy could maybe shed some light on that. But the hardball metal, or wheelweights, or even a mixture of range scrap with a significant percentage of hardball metal in it, will harden up considerably to the point where your big fifty can shoot through very large trees and the bullets will still look quite nice. They're not as hard as steel jacketed solids or turned brass bullets, but there's not much living on this planet that can stop them and they are hard all the way through. Great fun to go bust big rocks into little ones with!