My concern with the BHN of the projectle is the way the skirt on the minnie is expanding to engage the rifling of the barrel. If the lead your casting is too hard, I am wondering if the skirt is opening up enough to do the job.
As for ways to test the hardness of lead, take an equal amount by volume of a known lead and an unknown lead. Drop them into an equal amount of water, and the heavier of the two should show in the water level that rises to compensate for the lead. Lead being the heavier should make the water rise more.. should it not? I believe this was the archimedes principle. Therefore your current casted minnie VS a known pure lead minnie out of the same mold should be equal if the two know substances are equal themselves....
Someone also said you can take a known lead and using a vice exert equal pressures on a steel ball bearing and then measure the dent. The lead being the softer metal should dent more....
I took my .58 caliber out today to test different lubes that I make as to how they would effect accuracy. These tests were conducted at 50 yards off a bench. The rifle is a 32" open stock sights, Green Mountian Barrel shooting .570 Speer Round ball and a .018 pillow tick patch. I am rather happy with the results of the moose milk lube that I make as far as accuracy. This is a four shot group. Believe me, that little black square looks very small at 50 yards. That group is under an inch center of ball to center of ball. Actually it is closer to 3/4 of an inch group. I was very pleased.
Again, I encourage you to come to that other site with your concerns and questions. Not only would we enjoy having another person on the forum, but they are much more qualified to answer your questions then I am...
Until then keep trying combinatons. I sure wish I could help you out....