Doc, you wrote:

While on a casual first glance, both the Corbon DPX and Federal HST could initially appear to have larger expansion, the actual measurements demonstrate that all the recovered .40 S&W projectiles in the photo above, in fact all have exactly the same RD of 0.64". Also, be mindful that RD measurements are made at the leading edge of the projectile, not further back down the expanded "petals", as this can give a false reading. For example, in the photo above, when measured half-way down the expanded projectile, the widest part of the expanded "petal" measures 0.84", much larger than the calculated RD of 0.64".
Why are the smaller numbers used for analysis? A broad head arrow tip would appear little more effective than a target tip if average diameter is used, but that is clearly not the case. Is the interaction of bullets with living tissue so different from arrow tips that this analogy is meaningless?