Anyone can shoot a 9mm more rapidly with combat accuracy than a .40 or .45 of the same size and weight and with the same grip.
What's so true about this statement is that
in the world of shooting competitions, the
9mm has grown in greater use simply
because the shooters have learned
they can hit the targets, stationary and
moving, with more precision coupled
with greater speed.
So-called power factors have less
meaning.
Bill Wilson of Wilson Combat has commented
numerous times how he become more
competitive after a friend turned him toward
use of the Beretta 92 which was among the
principal 9mm guns at the time.
And I believe that when the U.S. military
adopted the Beretta, which was similar in
size and weight to the 1911, recruits become
more proficient.
Putting it another way, it takes a lot more
dedication to a shooting regimen to handle
the .45 than it does the 9mm. And most
LEOs or federal agents or civilians who
have guns for defense are simply. not that
dedicated or physically able to handle the
harder recoiling calibers.