One of the most confusing things about guns are how they are classified.
I'm not even getting into "assault weapons" and nonsense like that, I'm referring how you describe the type of bullet they shoot and what that means in practical terms.
Part of the reason for this is that firearms - particularly handguns - are limited in terms of what is practical to use. No one wants to haul around a wheeled carriage just for a sidearm.
Put another way, human anatomy puts a limit on about what diameter of barrel you want to use.
The diameter of the barrel dictates the size of the bullet. This hollow space inside is called "the bore," (as in they took a solid metal pipe and "bored through it" to make a gun barrel).
The measurement of this space is what is called the caliber. For example, the bore on a .38 caliber weapon is .38 inches.
I chose that example for a reason, because that wonderful measurement is hugely popular with gun designers.
The picture below is of three .38 caliber bullets. All of them are exactly the same width. No two of them can be used in the same firearm.
They are (from left to right) .380, 9mm and .38 Special.
Just for added fun, that .380 is also known as (9mm Short).
As I noted above, the width of the tube is pretty much the same for all three of these calibers. The difference is in the amount of powder (which means energy) used to propel them.
Look at that tiny .380. It is nice and small, and that means when you shoot it, there will be less energy expended.
The plus side of that is that the pistol that fires it won't jump around very much. In all honesty, none of these cartridges is particularly punishing. Even the .38 Special is considered fairly easy to shoot.
My point here is that caliber not only describes the width (and thus size) of the bullet, but also the power behind it.
All three of these cartridges are fired through handguns of moderate size. By way of contrast, the photo below includes two rifle cartridges.
Pretty big difference!
I want to draw your attention to the second cartridge from the right. That is a 5.56mm round for an AR-15. (Just to keep things confusing, it is also known as .223.)
The AR-15 is the civilian version of the dreaded and all-powerful M-16. Yes, the dreaded Assault Weapon!!!
How many times do you see AR-15 described as "high-powered, military-grade assault weapons?"
The picture above should give you some perspective.
Towering over the 5.56mm round is a .303 British cartridge. That rifle was invented more than 100 years ago and was the standard British Army weapon for World War I and World War II.
It was not unique - all the armies used a bullet that was almost identical in hitting power and range.
There are thousands of those kinds of rifles in private hands. In fact, they are quite inexpensive - much less expensive than an AR-15, for example.
The problem with them as that all the power takes a lot more practice to control. That was one reason why the US Army (and just about everybody else) switched to a smaller cartridge. Another reason was that smaller bullets meant you could carry more of them, making it less likely for you to run out of ammunition.
Look at the picture again and this time focus not on the size of the overall cartridge, but on the bullet sitting on top. If you look carefully, you will notice that while the rifle cartridges are longer, they are actually narrower.
Think about it: That .303 British cartridge is (by definition) .303 inches in diameter. That makes it smaller than the three handgun rounds.
Yet it is unquestionably more powerful. Again, this is because the powder behind it is so much greater.
It is very easy for novices to get confused by all the calibers, particularly because experienced shooter often develop very strong attachments to certain calibers.
There is a substantial community out there that insists that any pistol that doesn't have a caliber starting with a "4" is useless.
By this they mean that if you shoot someone with it, it won't prove lethal (or even incapacitating).
This is nonsense.
No one likes getting shot. Even a tiny .22 caliber bullet can kill instantly if it hits the right place.
Once I was somewhat sympathetic to the caliber fetishists, but I've since mellowed quite a bit. Basically it comes down to this: The power of the bullet is unimportant if you can't hit anything with it.
Smaller caliber firearms (both long rifles and handguns) are easier to shoot. They make less noise and have less recoil. That not only makes them less distracting but also encourages practice, which is essential.
For people with smaller hands and less physical strength, smaller caliber handguns make a great option.
They are also lighter to carry and easier to conceal. I know several people who initially used a beefy .45 or even .44 magnum as a carry weapon before getting tired of the weight and awkwardness of the thing. It is easy to look down one's nose at a .380 or .25 auto as a "mouse gun" but given the choice between having a 'purse pistol' and nothing at all, the purse pistol wins.