See the exhaustive FBI handgun study that was released a couple of years ago. It was the conclusions of this study that convinced the FBI to go back to the 9mm. There is an excellent two part summary of the study on youtube that you should be able to find with a search on the youtube search engine. (that's how I found it). It is very enlightening. The basic crux is that the concept of greater "stopping power" of a .40 cal or a .45 cal is a myth. The key for a handgun (which is an intrinsically defensive weapon) is being able to fire accurate, fast follow up shots and all of the FBI shooters from the best to the merely adequate were able to place quicker, more accurate follow up shots with the 9mm.
Except the FBI can carry highly expandable JHP’s.....
Military only carries FMJs....
Numbers are way different then....
The FBI studies were based on shooting jello and on an estimate of adequate penetration for civilian law enforcement purposes. The underlying assumptions and techniques my not apply to military needs.