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

Because the bullets are 5.56mm and not 6mm? It’s a measurement.. not a random number?

Or do you mean “why don’t they just manufacture 6mm bullets instead of 5.56mm?”