What Brinell hardness should I be at for a 9mm 124gr-125gr round nose cast bullet that I want to push to 1000-1010fps? I have found ones in a hardness of 10, 15 and 18.  Thanks.

The answer to your question will vary depending on the exact size of the cast bullet and the exact size of your bore. You will need to slug your bore and measure with a micrometer and then measure your "as cast" or "post sized" bullet .

Some people think that the speed is the only metric connected to how fast you can push a bullet. The truth is having a properly sized bullet is just as important as the hardness.

jonblack

IIRC, the commercial ones I've shot were around 15 BHN. I got a little bit of leading, but as the previous poster stated, size is as important as hardness(sometimes more, actually). I haven't shot any of my own cast bullets and I have no clue what the hardness of the alloy is.