Here's a (very) quick test, rotating in two axes: longitudinal, as if the bullet was fired from a weapon with rifling in the barrel, and; tumbling end over end.
Note that the longitudinal rotation is not easy to see in this video, a better camera position and taking the time to carefully set the rotation speed would improve this. Rotating in the third axis as well as the first two results in an unnatural oscillating wobble, I believe the 2-axis spin is a more realistic representation of a flying bullet. This uses the basic rotation script applied to the bullet's orientation controller:
float $speed;
$speed=6.3;
$orientation = (sin($speed*time),0,0,cos($speed*time));
The bullet element assigns a target object as its parent, and this parent object is further rotated using the same script, modified as below:
float $speed;
$speed=5;
$orientation = (0,0,sin($speed*time),cos($speed*time));
Note that the $speed variable adjusts the speed of rotation, and the position of the 'sin' calculation (either the first, second or third parameter of the $orientation function) selects the axis of rotation.