I was always told the reason the feds went to the 10mm round was because it didn't possess as much of an overpenetration problem of other smaller rounds,consequently making it safer for other agents who may be on the other side of a perp.
Virtually all of the heavier loads of 10mm tend to overpenetrate. They went to the 10mm because 9mm didn't work in Miami, either in parabellum form, or sub-magnum .357 (read, .38 +P). The .40 (I'm not saying it's perfect, it is still a handgun round, after all) is a refinement on that - the .357 Sig is (arguably) a further refinement.
The cartridge manufacturers are constantly working on this balance - bullet weight, muzzle velocity, terminal velocity, reliable expansion, "optimal" penetration, to find a good combination that works for various people - and they do a good job. But working with animal wounds is a thing of the past. Gelatin can test some things, but it has its limitations.
We don't really have enough data to judge these things until cops have shot enough people with them - that's the sad reality.
This post was edited on 12/27 at 1:55 pm