Cast Bullet Quality

[mdi]

As long as you get the bullets that are at least .001"(up to .0025ish) over bore size harness won't make much of a difference. 12 bhn will shoot the same as 24bhn at the same fps. The only place where I've found harder is better is with certain caliber the harder grips better without skidding when spinning up and on shallow rifling.
I have some .45acp 150gr lswc that accuracy went up when I added more lino to my alloy and ice water dropped them.The 45 barrels tend to have rather shallow rifling.
9mm "can" benefit with harder alloys due to the twist rate and getting the bullets spinning. Either way leading isn't an issue whether harder or softer.

I'd go with MBC and make sure the bullets are big enough and will still chamber, usually .002" oversize.

Yep! Bullet fit is way more important than BHN. If a 22BHN bullet is too small, it will lead at any velocity. If a 10 BHN bullet fits the gun correctly (fits chamber,chamber throats, barrel groove diameter) it will shoot clean to higher magnum velocities.

FWIW; I loaded some wheel weight alloy 158 gr. SWC over a too high load of True Blue for my .357 Magnum (an oops! on my part; .7 gr. over listed max.). Shot six in my revolver and when I opened the cylinder, the primers fell out. Way too hot! But when I checked/cleaned the gun I found no leading. Bullets were sized to fit the cylinder throats. Estimated velocity, 1600+ fps...