This debate has been going on for at least as long as I have been aware of guns, and it’s likely to continue long after guns have been replaced by phasers or whatever. At which point it will be just as stupid and pointless as it is today.
Bullet design has come a long way in the last 30-40 years. Hollow-points have become hugely more reliable in terms of expansion and penetration, and 9mm has benefitted from this a great deal. Perhaps this argument was more relevant in the 1980’s when hollow-point expansion was much more hit-and-miss? You might think so… but you’d be wrong.
In the 1980’s Evan Marshall took the radical approach of looking at actual, documented real-life shootings to determine what worked. This study was flawed by focussing on one-shot stops, but it was the first public scientific study of real-world shootings. (I call this approach flawed because, as one Marine quipped during the recent war in Iraq, “Who shoots them once?”) When comparing 9mm ball and .45 ACP ball he was rather shocked to discover that there was no significant difference in their ability to produce one-shot stops- and neither was all that good at it. This is not anecdotal evidence, war stories or what have you- this was documented in actual shootings.
The Miami shootout of 1986 prompted the FBI to adopt first 10mm, then .40 S&W. But it also launched a thirty-year comprehensive study of handgun stopping power, which reached the conclusion that handgun stopping power sucks. What matters is breaking things the suspect cannot operate without. This means that the bullet has to penetrate deeply enough to reach those things, and you have to be accurate enough to hit them. Everything else is icing on the cake. Well… almost everything else.
More recently Greg Ellifritz ( https://www.activeresponsetraining.net/ ) did a study of real-world shootings, and it does seem to indicate that caliber has some importance, but the difference isn’t heavy & slow vs. small & fast. All calibers had instances where they simply failed to stop an attacker with any number of hits, but common calibers below 9mm/.38 had this occur significantly more often; far, far beyond the statistical margin of error. That’s .32, .25 and .22. 327 Magnum and 7.62 Tokarev may buck this trend, but it seems there was insufficient data to determine this.
Counter-intuitive as it may seem, for calibers .380 and above, there was no statistically significant difference in stopping among the calibers surveyed- which included all major commercially available calibers chambered in defensive firearms.
This is not to say that no one should carry a .40 S&W, .45 or what have you- just that you shouldn’t do it with any expectation that the caliber will give you a margin for error. No matter what you are shooting you need to be able to place your shots where they will do the most good- and need to fire a bullet that will penetrate deeply enough to break the things it needs to- those being the heart, spine, aorta and brain.
So everyone should carry a .380? Not at all. On average there is little difference between these rounds, but conditions differ. Can you reasonably expect your assailant to be wearing heavy winter clothing? Do you need the ability to deal with dangerous animals? In these cases a more powerful round with better penetration may be advisable. In a very hot climate you might need to carry a smaller handgun- but this is balanced to a degree by the fact that people wear less (and thinner) clothing.
So caliber is less important than previously thought, but that’s no reason not to ‘stack the deck’ by buying high-quality, modern defensive ammo. In this day and age information about bullet’s terminal performance abounds; Youtube can provide FBI-style test information about all of the common bullets used in modern defensive ammunition. Pick a good one in the caliber of a gun that you like, shoot well and, most importantly, will actually carry and likely you’ll be just fine.
Michael Tinker Pearce, 30 June 2019