It's funny... as caliber wars become more pointless, they become more vocal and partisan...
This seems to actually be BECAUSE the data show the differential between chamberings is small, and has been getting smaller for decades. As datasets get larger, data and data analysis get better, and as ammunition gets better, the differences between common defensive pistol chamberings are progressively smaller, and less meaningful.
Pistols are Pistols, and Rifles are Rifles.
We have a saying "pistols are pistols, and rifles are rifles"; which means that the difference between reasonable defensive pistol chamberings is small, and effectively meaningless, compared to the difference between pistols, and long guns; both shotguns and rifles.
Compared to rifles, pistols are lost in the noise... and looking at the variability between different rifle chamberings, the differences can be greater than most powerful reasonable defensive pistol chamberings, and nothing at all.
In fact... if we just look at muzzle energy numbers, there can be greater variability within a single chambering.. say, something like subsonic .300blackout, and high velocity .300blackout, with something like 900ftlbs of energy difference between them, and the most powerful reasonable defensive pistol chamberings, such as hot and heavy .357 magnum, at something like 750ftlbs.
Pistols are pistols, and rifles are rifles.
When there were thought to be larger differences, one could have clear advantages and disadvantages which could be argued. When the differences are small, it comes down to preference, individual performance, and small optimizations. Now, there can be no clear differentiation.
Because pistols are pistols, and rifles are rifles.
So, the caliber wars rage on
The most recent "major shakeup" has commenced, as many law enforcement agencies ( who moved to .40 and .45 in the late 90s and 2000s, after a series of notable failures in effectiveness of 9mm in the 80s and early 90s) have decided to move back to 9mm.
This is happening, because both ballistic testing, and empirical shooting data, show very little difference in effectiveness with modern high performance ammunition.
9mm has always had a few fairly significant advantages... and these are not controversial... that were thought to be offset by its reduced effectiveness compared to .40 and .45. It costs substantially less, it's smaller and lighter (more rounds in the same size, or smaller and lighter with the same round count); and smaller officers, and those who are less skilled with firearms, and who practice less, generally shoot better with it.
Since police generally open carry, large "duty" pistols, and in general are not good shots, who don't get to train very much, and don't have much money for ammo when training; and because there are more and more women and smaller officers on police forces, 9mm is at its greatest position of advantage, in police duty pistols.
With the data showing that modern high performance ammunition gives little or no effective difference between 9mm, .40, and .45; there is really no reason for police duty weapons NOT to move back to 9mm, and some good reasons to do so.
And so 9mm partisans in the non-police shooting world, believe they have scored a final victory over the dark forces of the .45acp.
... this war having been going on literally for over 100 years now by the way; the chamberings having been been developed in 1901 and 1904 respectively, and having first fought on opposite sides of world war one...
Of course, the police duty carry mission, is different from the many different concealed and open carry missions that non-police defensive pistol carriers have.
So the "decisive victory" is nothing of the sort... it's just another set of data points.
There's caliber wars, and there's actual reasonable argument with data.
Most people wouldn't know what the second is, never mind the third.
... Presuming modern premium defensive JHP ammunition, without restrictions...
Basically there is between a 10%-20% or so spread of expected effectiveness between .380+p and .45+p, depending on exactly which data you look at, presuming a human in standard street clothes, and no barrier penetration. About a 10% spread from 9mm (standard pressure) to .45+p and less than 5% from 9mm+p to .45+p... so...
When you factor in things like heavy clothing, barrier penetration and the like, the numbers get even more murky, and less reliable, and less useful...
Effectively, the differences between reasonable defensive chamberings don't matter.
20% is worth debating... 10% maybe... but not really too much, 5% really not at all. It's margin of error, a slight optimization, or a personal preference.
Is it worth upgrading from .380 to 9mm...?
...Sure, if you can shoot it just as well, and carry it just as well...
Is it worth "upgrading to" .45...?
...Sure, if you can shoot it and carry it just as well...
...and you don't mind 2 or so fewer rounds in the same size package.
Is it worth "downgrading" to 9mm...?
...Sure if you can shoot and carry it just as well...
...and you want the 2 or so more rounds in the same size package.
What about .40?
It almost exactly splits the difference between .45 and 9mm, with none of the advantages of either, and all the disadvantages of both really... With modern ammunition, it's really no longer worth considering (except as a smaller version of 10mm, or the basis for .357sig), except "because I feel like it" or "because I shoot it well"... But it's also probably not worth trading or selling it for either 9mm or .45acp if you like it.
Functionally, the spectrum of defensive chamberings between 9mm+p and .45+p make no difference.
So... what DOES make a difference, and how much?
Ehh... not much, and not much...
Basically, a tiny bit with .357sig and a slight advantage with .357magnum and 10mm... and I don't consider .45lc and and .44magnum to be reasonable defensive chamberings for most people (they're effective, but they're bigger, more expensive, harder to shoot, and harder to carry than most will ever bother with).
There's a little bit of an advantage to .357sig, light .357 mag, and light 10mm over .45acp +p... maybe 5%.
There a slightly bigger advantage to medium .357mag and medium 10mm... a little more than 5% to maybe 10%
There's a slightly bigger advantage to hot .357mag and hot 10mm... a little more than 10% to maybe a little more than 15%, or even 20% under some circumstances (particularly with barrier penetration, and with tougher animals than humans).
However, for all of those upgrades, you get higher cost, more recoil, and its more difficult to shoot well...
In the case of medium and heavy 10mm and .357 in lighter easier to carry guns, it can actually be physically painful to shoot, and most people can't shoot them well at all.
Is it worth the upgrade to magnum class chamberings like these?
Sure, if you can carry and shoot them just as well, don't mind the extra recoil and the extra cost.
Getting the picture here...?
It's not about the chambering... because pistols are pistols...
It's really about how well you can shoot, and carry the gun, in your chosen chambering... and in personal preference, and in small optimizations for particular missions.
So, what do I choose? What do I recommend?
I'm a really big guy, who can carry almost any sized gun reasonably comfortably. I live in a rural, cold, northern state, and at times have had to deal with wild animals.
Most importantly for my chambering selection, I shoot heavy recoiling guns pretty well, and the differences in recoil between 9mm+p and light .357 and 10mm are basically meaningless to me; with only a slight disadvantage to medium and heavy .357 and 10mm loads.
My pocket carry guns are a 5 shot .357 revolver, and a 7 shot .380... I like how both of them carry, and I can empty them both into a 4" or smaller circle at 10 or less yards with deliberate fire, and an 8" or smaller circle as fast as I can recover each shot... though neither have much in the way of sights.
Once I got the small .357 revolver, I pretty much stopped bothering carrying the .380 frankly, because I can shoot the .357 as well, it's no heavier, it carries just as well, and it's more reliable. I occasionally carry the .380, particularly in shorts, because it's a little flatter and slimmer... but that's really it.
My belt carry guns right now, are all 3-5" barreled 7-9 round .45 and 10mm 1911s... But at times have been 7 to 16 round 9mm, .40, and .357sig, and .357 magnum I've never felt insufficiently armed with any of them. I have never felt over armed with any of them. Though a couple have been a little bigger or heavier than ideal.
I don't feel that an 8 round .45 1911 is significantly more or less effective, for my personal defensive needs and missions, than a 13 round 9mm Browning hi-power. I happen to own a couple of good 1911s in .45 and 10mm that I like... I happen to have sold all my 9mm pistols a few years ago, including my 2 BHPs, expecting that I'd buy more, but I haven't been in a position to do so.
The only ones that I thought were in any meaningful way more effective, were the .357 magnums, and the 10mm, in both of which I carried loads suitable to heavier animals than people, because I lived in the north Idaho mountains. If I had to dispatch an elk or a moose, or god forbid deal with a bear, I wanted 158gr to 180gr at 1250-1350fps... The biggest advantages I could get in pistols that I felt like carrying every day, and could shoot well.
... and really... it's only a small advantage...
Because Pistols are Pistols, and Rifles are Rifles.