DRG,
The two kills with on shot has only happened twice in three test runs of a 1992 scenario {US vs Iraq} with the Iraqis having over 200 tanks. So this as you said is not common.
The A-10 30mm has yet to make a kill on a tank. It will kill most everything else but no tanks yet.
I am asumeing that the 69mm pen figure for the gun is with normal AP. I belive that this weapon also has a DU round for it. If Im wrong then we all know what asume stands for!