I honestly have no idea. I suspect it is intended to be set on a trajectory and continue on it unless the trajectory is reset, but I could be wrong about that. I haven't used bullet except for enemies, and in one case where I redirected the bullet's angle based on mouse clicks. I used the other way for climbing ladders in a platformer and knew it would allow a change in X as the Y changed.
If char has a bullet behavior you could change char.bullet.angleofmotion when the arrows are clicked and the
char would look like it was changing lanes gradually.
char -> set bullet angle of motion to clamp(self.BulletAngleOfMotion - 10, -110,-70) degrees
for the left arrow and
char -> set bullet angle of motion to clamp(self.BulletAngleOfMotion + 10, -110,-70) degrees
for the right arrow.
I don't think that's what you're looking for, though.