Because the minor diameter of the barrel is 5.56 mm and the major diameter is 5.69 mm. If the bullet were smaller than that then the propellant would blow past it. They didn’t make a 'murican millimetre like they did with the imperial system.
In point of fact Americans have gotten impressive results out of far more complicated metrics than metric. It’s not a matter of understanding, it’s a matter of pride. And of not having to buy all new tools.
Americans cannot understand any metric
2 liter bottle.
Checkmate, athiests.
9mm
We’re more familiar with 5.56x45mm thanks to all our school shootings thank you very much.
In the same way a US ton and a metric ton is like 10% different, a 556 bullet is actually 5.7 mm across.
Because the minor diameter of the barrel is 5.56 mm and the major diameter is 5.69 mm. If the bullet were smaller than that then the propellant would blow past it. They didn’t make a 'murican millimetre like they did with the imperial system.
28 grams to an ounce
In point of fact Americans have gotten impressive results out of far more complicated metrics than metric. It’s not a matter of understanding, it’s a matter of pride. And of not having to buy all new tools.