Spin Rate

Spin rate is the rotational speed of the bullet, usually expressed in revolutions per minute, and it is what keeps a bullet point-first against the air. You can estimate it by dividing muzzle velocity by the twist length and converting units, which means it rises with faster ammunition and with a quicker barrel. A modern match round commonly spins somewhere on the order of a couple hundred thousand revolutions per minute as it leaves the muzzle.

Two inputs set the number: the twist rate of the barrel and the muzzle velocity of the load, with a gain twist barrel accelerating the spin as the bullet travels its length. The resulting rotation determines the stability factor: too little spin lets the bullet tumble, while a great deal of spin holds it steady but can amplify small effects like spin drift. Matching spin rate to the bullet is the whole point of choosing a twist.