For example, why 5.56mm and not just 6mm? Why 7.62mm instead of just 8?
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The real ELI5 answer is that they that the sizes were either based on old imperial measurements (which are unlikely to translate to round metric numbers) or were calculated to be the ideal size (which is unlikely to be a round number for anything).
Performance/efficiency is far more important than having a number that’s easier to write down.
>5.56mm and not just 6mm
6mm exists already, in the .243 Winchester and 6mm Remington, as well as many others.
>Why 7.62mm instead of just 8?
Same here. 7.92mm Mauser is frequently called 8mm Mauser, and there are other 8mm cartridges as well, like the Lebel and Remington & Weatherby Magnums.
Very minute differences make pretty big differences when the chamber pressures on modern firearms can exceed 40,000 PSI. Having more granulatity in naming also tends to lead to less instances of someone putting the wrong ammo a gun, usually, although accidents and stupidity happens.
There’s a story behind every round.
.30 Caliber rifle rounds were standard for many years. In metric, that’s 7.62mm. Quite simply, it’s a standard sized round that’s converted to a different unit.
5.56x45mm NATO was created to meet the CONARC parameters. They built a round to meet parameters and it just so happened to come out that size.
Remington was making the .222, Springfield made the .224 based on their design. Remington’s .222 underperformed, so they made a “.222 special” with a bigger case. It’s common in the ammunition world to give hotter loads a bigger number, so they re-named it a .223 (despite it actually being a .224 projectile). Converting the .223cal to metric, we get 5.56mm.
There’s plenty of “round number” bullet sizes, 9mm is one of the most popular in the world for instance. .30 cal, .40 cal, .50 cal, 10mm, and 13mm are pretty common as well.
Ballistics can change drastically as the size, shape, weight, powder charge, etc change. There are both 6mm and 8mm cartridges, and they perform differently than 5.56mm. New cartridges are generally designed around ballistic parameters, and the end result uses whichever bullet size and case dimensions meets that need. Sometimes it’s an existing bullet and sometimes an entirely new bullet is engineered.
Historically the caliber size started as a decimal of an inch, which makes sense, and we’ve just stuck with it. .50 for example is a half inch diameter bullet. It has gotten a bit convoluted over the years, as others have pointed out with the .357 and .38 special using the same bullet and .223 using a .224 bullet for example.
Don’t get me started on .30 bullets. There’s a lot of them.
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