556 was the measure of the inner diameter of the rifling of a barrel of a gun that shot 556.
Metric is confusing. That’s why for most shotguns, we measure the width by the number of lead spheres of that diameter that would equal one lb, eg a 12 gauge shotgun is the diameter of a 1/12lb sphere of lead.
Nobody knows how big 18.53 mm is, but everyone knows what a 12 gauge shell looks like.
Oh, and gun powder is measured in grains, maybe early smokeless pellets were about the same size as grains of wheat.
Metric was too confusing for bullets, so we use both, and but neither of them are actually the diameter of the bullet, most of the time.
.223" is the same diameter as 5.56mm (which is 5.7mm across), but if you use 5.56 in a 223, it might kill you.
223 in 556 is fine, might fail to cycle.
then why is it called 556 if its actually 5.7?
556 was the measure of the inner diameter of the rifling of a barrel of a gun that shot 556.
Metric is confusing. That’s why for most shotguns, we measure the width by the number of lead spheres of that diameter that would equal one lb, eg a 12 gauge shotgun is the diameter of a 1/12lb sphere of lead.
Nobody knows how big 18.53 mm is, but everyone knows what a 12 gauge shell looks like.
Oh, and gun powder is measured in grains, maybe early smokeless pellets were about the same size as grains of wheat.