I think that 556,xxx is a bit high for a blued finish. I not going to say it is impossible, just unlikely. I think that if you look at the finish in bright sun light, it will lack the bluish tint that a black oxide finish should have. I can see the two tone finish on the right side of the receiver, and the lower portion is almost a black color. But the top of the receiver is that dark color with a slight greenish tint and that is a typical Inland phosphate finish.

The barrel and receiver should have the same finish. If the barrel were blued, that portion from the gas cylinder to the breech will be a shiny, dark blue color, almost like a sporting rifle. I don't see that in the photos.
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