.223 Remington 55gr FMJ American Eagle

Same here. I don't shoot steel cased ammo. But its because its super dirty blowing back around the case that does not expand enough and you have to clean your gun more often than with brass or nickel or it will start jamming. You can keep it going for a while by keeping the bolt lubed or adding some oil so it runs wet but eventually it will have to be cleaned. The other reason is between the excessive fouling and the bi-metal bullets it will wear the barrel out faster.

AKs are different. That Russian round has a tapered case so its easier to extract and the 5.56/.223 is a straight walled case and not as easy to extract. So AKs are made to shoot steel case. 5.56/.223 ARs are not. I shoot 5.56 (mostly M193) in my ARs for defense loads but I shoot a lot of .223 in them also for just practice. Thanks for sharing.