pew

Now- imagine someone has their jugular severed completely. They will likely live for 5-15 seconds before they bleed out. Hitting less major circulatory vessels- a person can literally take days to bleed out. So if you hit someone in the general arm or shoulder let’s say- even with a .50 pistol round using plain old JHP- they won’t die on the spot- and with care will likely live. But more importantly- they will have time and very possibly the ability to still be a threat- especially if they have a weapon too. 1 second is long enough to shoot back- but 5-15 is an eternity almost.

Hit the same attacker dead in the heart, or better the brain- they still will likely have enough time they COULD fire back- but they will go down relatively quickly. So the “stopping power” of a round comes more down to shot placement than caliber- however some rounds are better suited to cause damage that through pain and other harm make threat mitigation more likely, and have other benefits as well in ballistics.

But it’s physics. To “stop” or “knock down” a human being- the force to the shooter has to be equal and opposite the force to the bullet. In other words- games and movies where the guy gets shot and flies back through the air? You’d need to generate enough force from the round that the person shooting it would ALSO be thrown through the air when they fired the gun. Handguns particularly usually lack advantage in the wounds hey create. This is due to the size and power of their cartridges, and certain effects that you can generate like cavitation inside a wound channel with larger and or more powerful rounds than are practical or common for most popular hand gun cartridges.

Now- when we combine experience with science- there are lots of advantages to certain rounds. But one thing that gets thrown around a lot? “The military uses XYX round...” or “well then why do Seals/GSG/Etc. Use XYZ round?” Well... the military... any military... doesn’t always do the “best” things- and has a tendency to keep doi things that aren’t quite right or make no sense for long times- there are politics and egos and red tape and personal opinions and all sorts of things that factor in.

You also have to remember- most modern militaries don’t actually take infantry small arms very seriously. They are critical- but most major military doctrines rely on a combination of multirole support from artillery, air, armor, etc- and use small arms under certain doctrines such as suppressive fire and other things. You also have to remember that modern militaries mostly either picked rounds based on what they THOUGHT the future of war was based on WW2 experience, or based on what was convenient and available or cross compatible and simplified logistics. The militaries of the world have used many rounds- each had its advantages and disadvantages-

But above all else the military doesn’t lick its ammo based on defensive use- they pick it based on battle field conditions where you’ve got designated marksmen, riflemen, machine gunners, explosives, fire support and other tools and tactics as well as the fact that you’re generally never supposed to be alone or the only person firing. It’s a different scenario. There are also rules and treaties and such that CAN limit ammo selection.

As for elite military forces- they generally use the weapons and ammo and load outs that are tailored for a mission. They don’t have to have one weapon that does everything ok and nothing great. They can choose based on the anticipated environment and mission parameters what will best suit their needs. Most people don’t quite have that level of flexibility in what they carry day to day- let alone do they have the level of intelligence gathering of a special operations team being sent in- which sometimes is very little but will almost always be more than what you have when you leave the house.

Generally speaking- anything from .38-.45 is going to fall well within the realm of what will be a pretty decent defensive load out. In general- larger caliber rounds just do the job better in a handgun- but they also have draw backs. What it really comes down to is how you shoot. If you never practice with your weapon, or you develop bad habits, flinching, hesitation, limp wristing- you’d probably be better off with a smaller caliber. But recoil is subjective and also has as much to do with the weapon as the caliber or the round and how hot the load is of heavy the bullet.

If you only have one gun and you never shoot because ammo is too expensive or it hurts you- you’d probably be better off shooting something smaller and cheaper in a round so you can actually use it. You can put 20+ rounds into person and they can still walk. It’s happened- and drugs etc can magnify that. If you shoot like shit a bigger round can be a crutch- but you also usually hold less ammo so if you REALLY shoot like shit... the extra rounds in a smaller caliber may be better for you.

It’s... if gets complicated. There is no king of bullets. There is no one round that is best at everything. People spend their whole lives shooting and hunting and loading rounds and studying and gunsmithing and working up dope sheets- and never master the whole thing. If you buy any of the most popular calibers you’re fine as long as you can shoot half worth a damn. Just get GOOD defensive loads- not round nose feather weight ammo for plinking paper, and practice your heart out (you can use the paper punching ammo for most of your practice to save some coin...)

Tl:dr- there are a ton of variables that effect the wound created by a bullet of any given caliber- from the type of bullet to the weight of the bullet, the charge of powder, and so on. You can use tests to compare rounds but there isn’t a perfect test to simulate the human body from every angle and every path a billet could take through it. Worry less about the size of the hole and more about hitting your target, and getting it in a place anatomically likely to do real damage.

As a soldier I say your pistol is only good for fighting to your rifle.
As a medic I like having a sidearm I could draw while hovering over my patient.
As a private citizen I say the best defensive weapon is what you can easily carry and consistently hit your target with in the environment you need it.