Gerard Schultz
Well-Known Member
Edd, do not think of it as the twist rate that changes but rather as the bullet that has to be shorter or longer.
Gyroscopic stability (Sg) increases as the bullet goes away from the rifle.
With a short bullet, the Sg is higher from the start and the bullet will perform well at shorter ranges. The transition from flight to tissue determines whether the bullet will penetrate in a line that is an extension of the line of flight or whether it will turn or tumble in the target. At shorter distances the bullet does not have the ability to increase the Sg yet and it has to be sufficient from the get go. Such a bullet will then be too highly stabilised by the time it has to nose over a long range trajectory. It will not be tractable, the nose of the bullet will remain above the line of flight and spin drift will become unacceptable as it flies the down path of a trajectory. BC also becomes worse because the bullet presents more of it's side to the direction of travel.
With a long bullet where the Sg is just enough to give stable flight, if it strikes a target at short distance it turns and tumbles very quickly. At longer distances it has had time to develop a higher Sg but not high enough to be intractable. It noses over the trajectory and flies nose first along the bow of the trajectory. BC remains intact, spin drift is manageable and Sg is high enough to ensure good terminal performance.
The steps are therefore to know what the twist of a rifle is and to decide at what distance the target is likely to be. Then choose a bullet best suited to do the job.
Gyroscopic stability (Sg) increases as the bullet goes away from the rifle.
With a short bullet, the Sg is higher from the start and the bullet will perform well at shorter ranges. The transition from flight to tissue determines whether the bullet will penetrate in a line that is an extension of the line of flight or whether it will turn or tumble in the target. At shorter distances the bullet does not have the ability to increase the Sg yet and it has to be sufficient from the get go. Such a bullet will then be too highly stabilised by the time it has to nose over a long range trajectory. It will not be tractable, the nose of the bullet will remain above the line of flight and spin drift will become unacceptable as it flies the down path of a trajectory. BC also becomes worse because the bullet presents more of it's side to the direction of travel.
With a long bullet where the Sg is just enough to give stable flight, if it strikes a target at short distance it turns and tumbles very quickly. At longer distances it has had time to develop a higher Sg but not high enough to be intractable. It noses over the trajectory and flies nose first along the bow of the trajectory. BC remains intact, spin drift is manageable and Sg is high enough to ensure good terminal performance.
The steps are therefore to know what the twist of a rifle is and to decide at what distance the target is likely to be. Then choose a bullet best suited to do the job.
