Off the top of my head the only American developed round that’s designated in metric is the 10mm. But I think it’s only called that because it was supposed to replace 9mm for the FBI, but 9mm was developed by Germans. Nearly every round designated in millimeters was developed in Europe.
Edit: I just remembered 6.5 Creedmore and 6.5 grendel exist, but those draw their lineage back to a Russian parent case (5.6x39mm). So my point still stands I guess
Off the top of my head the only American developed round that’s designated in metric is the 10mm. But I think it’s only called that because it was supposed to replace 9mm for the FBI, but 9mm was developed by Germans. Nearly every round designated in millimeters was developed in Europe.
Edit: I just remembered 6.5 Creedmore and 6.5 grendel exist, but those draw their lineage back to a Russian parent case (5.6x39mm). So my point still stands I guess