One of the most misunderstood things about firearms is how they actually inflict harm. There is a lot of gun enthusiast jargon that purports to describe what bullets do when they hit something, but most are in fact just so much blather.
Sad to say, I personally used many of these pseudo-scientific terms, some on this site.
The fact of the matter is, no one is 100 percent sure exactly how ballistics work. There have been many, many studies on trauma and the findings are still in dispute. The big reason is that there are simply so many variables, particularly in combat situations. Some academics try to mimic this by firing shots into gelatin blocks or even using life (soon to be eaten) animals, but the problem here is that none of this tells you how a person will react, particularly if there is a surge of adrenaline (as would happen in combat).
This article therefore focuses on stuff we know, the absolute no-kidding basics about what bullets do. There are theories (usually pushed by ammunition manufacturers trying to sell their wares) but that is open to dispute.
As anyone who has ever seen a bullet strike a tin can or bottle of water knows, bullets strike with considerable force and this often inflicts severe damage on the target.
In terms of doing harm to animal life, this damage does two things:
1. It destroys the immediate impact area, which can lead to near-instant death if it is a critical area like the brain.
2. It causes blood loss, which also can lead to death.
That's really all guns can do. They don't fling people backwards, propel people into the air, you can't snap your wrist and make the bullet curve around a wall and generally you're not going to cause a car to explode by firing a handgun at it.
In fact, we can get even more simple: Bullets make holes in things they can penetrate and bounce off of thing they can't (though sometimes they will just lodge in it).
Now if you go and read up on guns (particularly for combat or self-defense use) you will get all kinds of nonsense about "knock-down" or "stopping power" or "over-penetration."
Only the latter phrase even has any actual meaning. The other two are just cool-sounding nonsense.
Let's deal with knock-down first.
The idea behind this phrase is that a bullet will hit so hard it will actually knock someone down. I believe the concept is that in combat, sometimes people who are hit by bullets fall down right away. (Sometimes they don't. Read some Medal of Honor citations for a number of amazing exceptions to this belief.)
Of course it is perfectly natural for people who are being shot at to duck down to the ground or otherwise take cover. Some do so while being unaware they were hit. The notion that someone can take even a large firearm and physically blast someone backwards, knocking them to the ground is pure Hollywood.
The closest thing that could happen is that at extreme close range the muzzle blast (say from a shotgun) might cause someone to flinch backwards and stumble, but there is simply not enough energy in a hand-portable firearm to physically hurl a 200-pound man through the air, much less cause one running at you to be blown backwards by the impact.
Can a cannon do that? Absolutely.
A hand gun? No.
But it does look cool in the movies so we buy into it.
A close cousin of "knock-down" is "stopping power." It's basically the same concept: the cartridge in the gun is so powerful it stops an attacker dead in his tracks. It even has its own wikipedia entry.
Yes, it is true that people may stop abruptly when the see a gun pointed at them and they may stop even more abruptly when it fires (particularly if it close to them), but this has more to do with their reaction to the flash, smoke and noise than the impact of the actual bullet.
Going back to what I wrote above, remember that bullets can have only a limited effect, even if they are big and coming very fast (say a .44 magnum).
They can injure and even kill because if they hit a vital area, they can destroy that vital area and without it, a person will die (hence the term "vital"). We are talking primarily about the brain and upper spinal column. If either of these areas are struck, there is an instant usually lethal effect.
The heart and major arteries are also vital, but it may take a few seconds for the effect to be felt. I witnessed a deer hunter score a direct hit on a buck, effectively obliterating the animal's heart. The animal was still able to run 20 yards before it expired.
Hits to organs can also be lethal, but they take much longer. Before the development of modern surgery, hits to the abdomen were feared because they usually caused a slow, painful death.
There are actually lots of places where one can be shot without significant injury (though obviously it still hurts). This is why the number of wounded almost always outnumber the killed in combat. People can survive quite nasty gunshot wounds and even fully recover.
If you happen to have clicked that link, you will note that it uses other jargon like "man-stopper" and offers various completely made-up definitions. That's exactly the kind of thing I am talking about. It is the worst kind of caliber fetish.
One of the big questions in terms of a wound being lethal is whether it causes a lot of blood loss. Without blood, animals die. One way to ensure that the animal loses a lot of blood is to make the largest possible hole.
To do this you need a lot of energy and a large bullet.
One thing about bullets is that the shape matters. More than a century ago firearms experts came to realize that a flat or hollow-pointed bullet would make a bigger hole than a pointed one. Bigger holes of course mean more damage, either further wrecking a vital area or causing blood loss.
This brings us to the third widely-abused term: Over-penetration. Any time a shot goes through a target, it has over-penetrated. This is generally not much of a problem unless there is something behind the target that you didn't want damaged (hence the gun safety rule about knowing what is behind your target).
There is a school of thought that the energy imparted into the target should transfer as completely as possible - any extra energy caused by the bullet passing through is wasted. Thus, over-penetration is a really bad thing.
In my youth I bought into this until I realized that the bigger the hole, the better. Period. A bullet that lodges halfway in an animal by definition isn't doing as much damage as one that cuts completely through. All other things being equal, you want that bullet to come out the other side as it offers more opportunity for blood to lead out (and can hit more vital organs).
This is not to say that all firearms are equal because they clearly are not. Nor am I going to argue that a .380 has the same lethality as a .500 magnum, because it doesn't.
What I am saying is that shot placement is more important that what you use. Missing with a .500 magnum is a lot less effective than hitting the target with a .380.
Closely related to that is the fact that lighter calibers are a lot easier to control, making you more likely to hit what you are aiming at.
I will also anticipate some comments by pointing out that I know a lot of military people and the controversy over whether the army should keep the 9mm pistol or go back to .45 is a valid one. Particularly in a combat situation with adversaries who might we bearing some sort of armor (or lots of gear that functions like armor) it is entirely reasonable to bring the biggest gun you can carry to the fight.
However, most of our readers are not probably not military procurement officials. They may own a firearm or want own one and are concerned about making an appropriate purchase. Or perhaps they are wondering if they should get something else because their current firearm seems a bit much to handle.
Hopefully this article has helped clarify things. I will close by pointing out that while lighter calibers are often derided, I have seen far too many articles on self-defense news sites where these supposedly inadequate weapons stopped an attack.