Furthermore, rounds like the .303 British, 7.62x54, and .308 all have very similar bullets but the powder charge, shape of the case, and type of primer all vary.
Technically speaking, the .223 is a civilian version of the 5.56x45mm NATO round.
When you have a designation in that nomenclature, it’s actually relatively simple.
The 5.56 refers to the approximate caliber of the bullet, give or take .1mm because of general silliness and lack of standardization.
The 45 refers to the length of the case, not the entire cartridge. This means that a 5.56x45 will always have a 5.56 caliber bullet, its case will always be 45mm long, but other features including the powder charge, bullet mass, bullet composition, etc. are all variable.
And then you get into the travesty of Imperial designations.
Technically speaking, 7.62x39 (what the AK-47 is chambered for), 7.62x51 (NATO standard full-size rifle ammunition), 7.62x54 (the Russian equivalent), .303 British, .308 (the civilian variant of 7.62x51), .30.06 (the parent of the 7.62x51 NATO which was developed to increase reliability in semi-automatic firearms), etc. are all about “.30 caliber” and are often referred to as such.
Then we get to the current travesty, .22lr vs .223 Remington. As you can see, they are the same caliber, that is the bullets have the same diameter. You can see that in the photo. However, there are many obvious differences.
First of all, the obvious size difference in terms of the case.
Second, bullet mass. Your average .22lr bullet is going to weigh between 30 and 40 grains. An average .223 is going to be in the ballpark of 55-80 grains.
Third, velocity and powder charge. A .22lr will get that 30 to 40 grain bullet to a velocity of between roughly 1000 and 1750 feet per second.
An average .223 has a muzzle velocity of between 2,750 and 3,500 feet per second.
What this means in the end is that a .223 is quite capable of having ten times more kinetic energy when fired than a .22lr. Comparing the two is no more ridiculous and no less than comparing that .223 and a .50 caliber machine gun cartridge.