That bullet didn't open and then tumbled because of it's resultant shape. Not opening put it's center of gravity too far back and then just like the normal FMJ would have done, tumbled. If the bullet would have opened, it would have been weight forward and stayed on course with "normal" performance. Why it did not open; I can't say.
Your 223 no doubt stabilized that bullet just fine in the Air but once it hits meat other forces and actions are required to stabilize it. Short version is: Must open up and create weight forward profile.
Are you sure it didn't tumble first, and not open due to tumbling?
