For those having access to them, the H&N Hi-speed bullets would IMO make quite nice and not-too-expensive fodder for such rifles - they have 350gr pills, and the bullets are made of quite soft lead with a few proprietary coatings, which allow them to be used at higher MV's than traditional lead slugs. The shape is kinda truncated-cone flat point. maybe another solution if you don't want to use jacketed bullets.
Here some infp from their website:

.45 / 70 KS

Diameter
Zoll/Millimeter .458 / 11,63 .458 / 11,63 .458 / 11,63 Hollow Point: .458 / 11,63

Gewicht
Grain/Gramm
300 / 19,44 350 / 22,68 400 / 25,92 300 / 19,44
Ausführung
HS HS HS HS