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

It’s not all that standardized. Some measure diameter of barrel, some measure diameter of the bullet, some are rounded to the thousandths, some to the hundredths, some purposely use a different number to differentiate themselves from a same diameter bullet.