For what it's worth, here is a link to the first part of an FBI paper on handgun wounding factors and effectiveness:
http://www.pointshooting.com/fbipart1.htm
As I understand it, the paper was the basis for the FBI's going to .40 caliber for penetration effectiveness.
However, 40 cal's, as I understand it, are "hotter shots" on a 9 mm frame, and which because they are hotter, are harder to shoot accurately.
In some tests I have made, I found that a 40 cal is harder to shoot "accurately" than a 9 mm.
Also, and as I understand it, advancements in bullet production since the paper was published, have resolved the penetration differences between the "old" 9 mm and the 40 cal.
So it may be that one dilema, was resolved by another, which was followed by a resolution of the original dilema.
At least the paper's origin is the FBI.
Just trying to be helpful.