For example, why 5.56mm and not just 6mm? Why 7.62mm instead of just 8?
In: Technology
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.
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