I have never had a problem firing 5.56 in a .223, only in reloading due to lower case volume in the 5.56. The article warns of lede dimension (I am guessing these are industry standards for MINIMUM dimension). You could chamber a 5.56 and eject it. If you don’t see any barks on the bullet from where it contacted the rifling in the barrel then you should be good to shoot it.
“barks” should be “marks”.
Great, thanks!