Hunting with cast boolits requires some deep thought. I've seen hardening by water quenching, super hard alloys etc. The truth is a combination of both hardness and velocity will net best results.
Any cast combination requires a boolit to expand at a predetermined hardness and velocity. I have found over many years that an alloy that is relatively soft with base protected by a gas check will work best. The 30.06 with a 210gn Lyman boolit when cast 1:30 and driven to 1740fps is an effective hunting round.
For plain based boolits a velocity in the 1100 to 1400fps expands well using an alloy of 1:40. As seen below, recovered boolits show perfect expansion. The little 80gn 25/20 was recovered from a head shot fox at approximately 80 yards.
L to R : 520gn 45/70, 250gn Long Colt, 420gn 45/70, 50 cal muzzleloader, 80gn 25/20