Twist Direction
Also: Right-Hand Twist · Left-Hand Twist
Whether the rifling spins the bullet clockwise or counter-clockwise, which sets the direction the bullet drifts laterally in flight.
Twist direction describes which way the rifling spirals down the bore: a right-hand twist spins the bullet clockwise as seen from behind, while a left-hand twist spins it counter-clockwise. The large majority of factory barrels use right-hand twist, though some makers and a few cartridges favor left-hand. It is a separate property from the twist rate, which sets how fast the bullet spins rather than which way.
Direction matters at distance because the spinning bullet steadily walks to one side, a behavior called spin drift. A right-hand twist pushes the bullet right, a left-hand twist pushes it left, and the effect grows with range and time of flight. Shooters who reach extreme distances build this bias into their dope, and the same logic carries into a gain-twist barrel where the rate accelerates but the direction stays fixed.