We see lots of opinions on it, lots of anecdotal experiences, and of course many of us who have already developed a preference for one or another cartridge are skeptical of the others.
Take a few of each shooter type:
- 'newbie'
'some experience but petite'
'some experience but strong'
'more experienced and prefers 9mm'
'more experienced and prefers 40 S&W'
'more experienced and prefers 45 ACP'
- slow-fire targets,
a single moving target, and
multiple targets requiring prioritization
So yes, if 'concealability' or light weight is a high priority on a given day, I'll opt for the 9mm (don't have a 40), but the real question is....what about the days when I am fine with a full-size CCW gun...?
I have a full-size 9mm in a Taurus PT-92, and I haven't even formally compared that to my 45 ACP 1911 in terms of the above scenarios, but if I did, it might reflect more difference between 'platforms' (love that word ) than the cartridge itself.
Anyway, it would be interesting. I'm betting some police departments or military units have actually done that experimenting, but of course their results might not extrapolate to civilian shooters or scenarios.
I would think that in 'combat' one could make a case for a less powerful round that penetrated well even if not causing immediate lethality, if the tradeoff were a much higher magazine capacity and/or much better controllability for multiple targets all at once. On the other hand, the average civilian shooter is (I think) more likely to face fewer bad-guys (most often just one I'm suspecting), and may therefore need the balance shifted less towards volume of fire and more towards stopping-power within their limits of accurate same-target followup shots.
If someone wants to donate three Glocks and a few hundred rounds of ammo, I'd be glad to complete the 'more experienced and prefers 45 ACP' category of testing towards this noble end...