Dusty let me say up front, I'm not an expert or do I play one on TV.
We sold thousands of bullets while operating Dry Creek Bullet Works. While we weren't Beartooth or Cast Performance there was a fair amount of game killed with our bullets ranging from deer to American Bison. We had no bullet failures brought to our attention.
Our standard bullet tested 15 BHN. IMHO this is harder than needed for what I consider idea sixgun velocities of 1200 fps. At this speed I mostly use straight wheelweights or 1-20. These test 9 and 10 BHN and work great.
Above this speed for rifles at 1600-1800 fps I like 15 BHN. It is soft enough not to shatter. It's ok for a cast bullet to deform somewhat but it should stay together and drive straight. The harder alloy at speeds above this are just to brittle IMO.
A call came this week from a former customer who had bullet failure across the water using a 22 BHN bullet. Not once but twice this happened on diferent animals using a direct shoulder shot with the PH calling each shot. Velocity was 1850 or so.
In a word, 15 BHN is plenty hard for anything I'll do with a sixgun.
God Bless
Creeker