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I've never owned an AR15, so I've only bought one semi-auto rifle.
I didn't say that the rounds "won't fit". I said "won't work", which is what I believed when I posted that. I had read about the cartridges a few years ago and assumed that the larger diameter throat of the NATO round meant that it wouldn't eject properly, but I was wrong. What I just learned from the link that I posted above is: "It is absolutely possible to fire 5.56mm NATO ammunition out of .223 Remington chambers with no issues.
However, combine ammunition that is toward the top end of acceptable safety limits with hot summer temperatures and a little too much fouling in the chamber, and you stand a good chance of running into some serious trouble. The rifle probably won't blow up, though it is possible.
More likely, it will blow the primer out of the primer pocket on the case and it will end up somewhere in the internal workings of the rifle. Obviously, this is not good and can result in a frustrating and dangerous situation."
So, to me, you should probably shoot .223 ammo through an AR15, but for those who like living on the edge, do what you want.
The AR15 will shoot 5.56 but it wasn't designed for the higher pressure of the 5.56 ammo.