Ken and John got it right, I had got my mass and BC's backwards. Antimony expands on cooling, and harder bullets from the same mold will get lighter as the antimony content gets higher.
The Ballistic Coefficient is a correction (bugger factor, Finagle's constant) to compare a bullet's trajectory to that of a standard projectile used in firing trials. It gets bigger with increasing length, and with mass for diameter (these two are a consequence of each other, given solid bullets). For a given twist, a longer, higher BC bullet is less stable. For a given load, a longer bullet will travel slower; I don't know if that makes it more or less stable. (Just went to JBM calculator - it's less stable.)
The ballistic equations will give you drop, time of flight, and change in velocity from tables, or on-line calculators. If you measure any of these you can determine the BC that will 'correct' your trajectory to the measured (or modeled) one,
I went at this from the back door; my 45-70 was sighted at 200 metres (we call it 40 rods ) and due to rain, and curmudgeonliness I shot it at 50 and 100 yards. I could then go to JBMs external ballistics, and found my trajectory and velocity most closely matched a BC of .4 . Then I calculated my sight settings, and found that my 3" tall tang sight should get me out to 1000 yds, and I'm only going to 800 so I should have some sight stem in hand.
Paul, your method is really ingenious and interesting, you're wringing out that Labradar. I can't find my copy of Veral Smith's book, but I'm sure he annealed, as well as hardened WW (antimony, arsenic, and tin lead alloys). That would give bullets of the same density from BHN 6-9 right up to 25 or more. Your method would be capable of sorting this out, where measuring drop lacks precision.(At least with the groups I can shoot, it does.)