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Does not the gyro effect enter into the external ballistics formulations? That was my assumption on the reasoning for rifling a round ball shooter.
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For a perfectly smooth spherical ball with no density variations, the gyroscopic effect wouldn't be needed. No matter how the ball rotates in flight (pitch, yaw or roll), the profile that it presents to the oncoming air would remain unchanged. Compare that to a longer, pointed bullet where if it yaws or pitches, the nose will no longer be pointed straight ahead and it will eventually begin to tumble if the nose gets too far away from "straight ahead".
What inspired my post in the first place was the total disconnect between the phenomena that are modeled in Greenhill's complete formula and the exterior ballistics of a round ball. On one hand, none of the characteristics that are accounted for in Greenhill's formula apply to round balls - since both the center of pressure and the center of gravity of a perfect round ball are always at the exact center of the ball, a perfect round ball can be neither dynamically stable nor unstable in flight.
On the other hand, the surface imperfections and density variations that make rifling useful for real-world spherical balls are things that are not accounted for or modeled at all in Greenhill's calculations.
In short, the fact that Greenhill's simplified formula "works" for real-world spherical projectiles is nothing more than a happy coincidence. Further, Greenhill's calculations only work for round balls if you use the common simplified approximation (TL = C * d^2). If you were to use his full formula instead of the simplified version, the equations would "blow up" and the result of the formula would be that you should use no twist at all (one twist per infinite inches of barrel length).