So from what I'm reading, it is perfectly fine to shoot .223 out of a 5.56 barrel and there are negligible effects on performance. The weight and grain of the barrel would affect performance and the twist rate much more.
So .223 out of a 5.56 marked receiver and bbl is just fine, but 5.56 out of a .223 mark receiver bbl is not. Matching proper twist rate and bullet weight is more important, but there is no harm to hte rifle shooting hte lower pressured 223 out of a barrel meant for 5.56.
Thats the way I understood it before, but I seem to have confused myself