For example, why 5.56mm and not just 6mm? Why 7.62mm instead of just 8?
In: Technology
To add to what others have said, the bullets weren’t designed around nice round numbers because a) they weren’t necessarily designed with modern units and b) they were almost all designed for military contracts, to be as cheap as possible while still being effective.
So while making them slightly bigger to get a nice round number might seem attractive, a government that’s buying ten million bullets isn’t going to want to pay for that extra material. Add this to the various ways scaling up or down can affect projectile performance and you can see why that sort of thing was never taken into consideration.
Latest Answers