.223 and 5.56 are not exactly the same. True, there are not many .223 rifles that shouldn't be used to shoot 5.56, but they do exist.
Colt has made many, many different versions of the AR-15 family over the last 50 years. Many are very different from their military M16 and M4 models. I still would assume all those barrels were rated for both .223 and 5.56, but I won't pretend I know that history well enough to say that for sure.
According to you, anyone who doesn't know the complete history of the Colt AR-15 line isn't smart enough to participate in the hobby of shooting. Well, I guess that includes me: I carried a Colt for my four years in the Marine Corps infantry, I've owned several Colts starting from when I was 18, and I sell them for a living now. But because I can't say for sure that they've NEVER produced an AR-15 style rifle that was chambered in .223 only, apparently I need to find a new hobby and a new line of work, huh?
0331: "Accuracy by volume."