Let me first qualify my ignorance by stating that I have never put steel cased 5.56 through my AR. This is more because I don't shoot it very often than any other reason.
But, while it is true that both case and barrel are steel, it is also true that probably no other material has the potential to its properties so vastly changed as does steel. The barrel in my Oly Arms is 416 stainless. Most AR barrels should be 4140 or 4150 chromium-molybdenum steel. This means that it is fairly hard.
I have seen russian 7.62 cases put through the spectroscopy test on a scanning electron microscope (yes, I know, not very accurate for light elements, but still informative), and am here to opine that combloc steel for ammo cases is about the closest thing to pure iron you will find in the industrial world. I don't think this stuff has enough carbon to qualify for 1003 grade. Big, round, clean grains of alpha iron translate into very soft steel that is likely not more than a few points HRb above brass. Realistically, if the stuff had any strenght, they couldn't draw it economically.
It wouldn't bother me any to shoot it in my rifle.
BattleRife