To add on what Raph said, Best way to combat a tank when using tank busting guns is to zoom in on the tank, if its turrets aimed at ya probably not a good idea to come in low and level, I usually line up and zoom then jerk away real quick.
If you are diving straight down on a tank, 99% of the time its not going to shoot back unless its an M4, or unless its angled on a hill, which in any case (and rarely it happens) I've shot down a 47 diving straight down on me.
Even when zooming it doesn't always work, sometimes I get to close before I see the turret then *poof*