How does the law of conservation of angular momentum apply to barrel rifling in firearms? I'm looking more for the effect it has on how the bullet flies.
If the rifle was not held or constrained, when fired, it would rotate in the opposite direction of the bullet traveling down the barrel. Its rate of rotation would be such that its angular momentum's magnitude would be equal in magnitude but opposite in sign to the angular momentum of the fired bullet. If the rifle is constrained the angular momentum would be converted to a torque.
The angular momentum of the bullet creates a gyroscopic effect via the law of Conservation of Momentum that stabilizes it from changing its angular direction while it is flying. The magnitude of the angular momentum is determined by the rifling twist rate and the bullets barrel exit velocity.