I noticed today that a M16 can't use .223 rounds. I do not find this surprising, but it is factually inaccurate.
Most 5.56x45 weapons can safely fire .223 rounds. You don't even need a different magazine. Not quite as good at longer ranges though.
A .223 firearm *can* fire a 5.56 round as well, but it is potentially unsafe since a 5.56 round produces more pressure that could in best case wear out the gun faster, get jammed up inside the mechanism, or worst case make it explode in your face.