I will agree the government adoption of the 223 Rem is a large part of its popularity. But, it is wildly popular and is likely the 2nd most popular round fired from a rifle.
If the 223 wasn't the adopted round for the AR-15, would the 222 rem or Rem Mag match this popularity? Hypothetically of course.
If the 30-06 wasn't adopted, would the 30/40 Krag still be the kind of the woods? Would we have a 270 Krag, 280 Krag, etc? Or maybe the 303 Brit would be the popular one...? Just wondering how things would have worked out had it all been different!
