Every video I've ever seen of the A-10 involved it doing nasty things to tanks with its gun, at the very least rendering the tank inoperative. From wiki, sounds like it uses 5:1 mix of ~1 pound AP and HE rounds.

Like greedo alluded to, MBT armor is usually concentrated in the front (and angled) to defeat rounds from other tanks fired from roughly on the level, they tend to skimp everywhere else, and especially on the top.