Why are bullet sizes usually in decimal and not just a whole number?

Question

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

In: Technology

Answer 1

There are some very good explanations of bullet development, but I feel like the simple answer is similar to why we don’t say that a 5’10” (or 1.778 metres) person is six feet tall (or two metres):

It’s not accurate.