system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:mm numbers and the power/lethality of a firearm

Is a 15mm more dangerous than a 9mm? What does .308 mean?
gpt
It's confusing because we don't use consistent measurements.

The caliber (like 9mm or .308) is the diameter of the bullet. Other things being equal, bigger is more powerful. A 10mm is more powerful than a 9mm, partly because it makes a bigger hole, but also because it's heavier and hits you harder.

Those are both measured in millimeters. For other calibers we measure in inches. Common ones include .22, .38, .357, .40, and .45. Any time you see a decimal in front, it's an inches measurement.

Everything I've listed so far is a pistol caliber. (Well, mainly, .22 is also used in rifles, and  any pistol caliber can be used in a "carbine" which is basically a rifle using a bullet mainly used for pistols.) By and large, bigger is more powerful, but an exception is that .357 is more powerful than .38. The reason is that .357 cartridges have a lot more gunpowder, so the bullet goes faster. More energy, more impact.

Which brings us to rifles. Most rifle cartridges have lot more gunpowder than pistol cartridges. You have that long rifle barrel, and with lots of gunpowder you can accelerate the bullet the whole way. If you used that much gunpowder from a pistol, a lot of it would burn after the bullet leaves the muzzle, making a big flash and a lot of noise but not pushing your bullet that much more.

So generally rifle calibers are a lot more powerful even though they might be smaller. The military uses .223, almost the same size as .22 but it goes way faster and will do a lot more damage. (Actually they use 5.56 mm, almost the same but with slightly different specs...but any 5.56 rifle can fire .223.) And of course you can shoot a lot farther with rifles.

.308 is a bigger rifle cartridge, better at penetrating cover and shooting long-range. It's the smallest rifle caliber commonly used by snipers.

Shotguns are completely different. The common sizes are 12-gauge and 20-gauge, and the 12-gauge is bigger. The measurement works like this: take a lead ball that just fits inside the barrel. How many lead balls do you need to make a pound of lead? That's the gauge.