However, I believe I might know the answer to your question, 44 AMP, if it is as simple as I am thinking; is it because the greater amount of momentum in heavier bullets keeps them going in a straighter line?
I'm not going to give the answer away either, but with regard to your hypothesis on momentum of heavier bullets . . . if you drop two bullets, one heavier than the other, at the same time from the same height, which hits the ground first? Or, if your gun barrel is parallel to the earth's surface (ignoring curvature) and you drop a bullet at the same time as you fire a bullet out of the barrel from the same height, which hits the ground first? The correct answers will illustrate one of the fundamental principles of physics and will also shed light on whether your hypothesis (above) is correct.