If I may,
They(the big three 9mm, .40S&W, and .45 ACP) are very close now'days. I agree with all the guys that say it depends on the platform the round is coming out of. If I can't have a long enough barrel to get some good velocity(for penetration) why use the 9mm? Like when I carry concealed(IWB), shorter is better, but detrimental to velocity(penetration). In this case, I choose .45 ACP, not because bigger is always better, but because it is less dependant on velocity for it's intended effect. This is the reason I will NEVER, EVER, get a 9mm pistol with a short barrel, you just took away the one thing you had going for it, speed. Now I also carry/use .40 S&W, shot out of the Glock 23(the mid-sized, OWB carry I can have a longer barrel). Since the barrel was going to be less than a full-sized, I wanted a little more ,"Oomph!!",behind each round when it hits. Which caused me to bypass 9mm, and head straight to the .40S&W! .40 S&W happens to be the best caliber for mid-sized hand guns,IMHO, due to its balance of this. The smaller the gun(shorter the barrel), the bigger the hole I want at the end of it; since a round with no velocity only has one thing going for it, it's size
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*warning* Slight Side-track to my soap box *warning*
This is also why my AR pistol has a 10 1/2" barrel instead of the really cool 7 1/2" barrel that is so popular. With itty bitty bullets, velocity is your friend. If I had to take a weapon with that short of a barrel, I would want it to be a Krinkov type pistol(AK pistol),because, like I said above, once you take away the velocity from a bullet, all that is left is the diameter of the hole it will punch
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I now return you to your regularly scheduled caliber battle goofines... I mean goodness lol...