I think you might have it wrong respecting expanding bullets, Richard.
What keeps the bullet traveling point on is its rate of spin. It is called gyroscopical effect, the same effect that keeps a peg top standing upright while it turns at full speed. When it starts losing angular speed it starts wobbling and when it goes below a certain threshold it falls.
And the spin is severely reduced when mass separates from the longitudinal axis of the bullet. The more mass, and the more it separates, or in other words the wider the mushroom, the more angular velocity it loses. Up to a point where it tumbles and dependng on the linear speed and what it hits it ill have more difficulties to keep straight on and will more likely bee to veer off its original course.
You have an example in ice skaters, that thing they do when they turn and turn on the same spot at full speed and want to stop suddenly. What they do? They open their arms to displace mass from their center of mass, lose angular velocity, and stop easily.
Alvaro
