STUMP49,
I totally disagree. Except for the throat length and taper the differences between the two are less than the allowable tolerances. The throat length is immaterial because any ammo that will fit in a magazine will never get close to either throat.
The headspace is the same just with a tiny bit more tolerance for the 5.56.
I am looking at the actual chamber prints when I say this not something copied and pasted off the internet.
The proof is the fact that 100s of millions of rounds are shot interchangeably every year with no reported problems.
Jack