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I am reposting this because it was not resolved when I first posted.

  1. Bullet spin causes a bullet to become a gyroscope. Specifically, bullets have their center of pressure in front of their center of mass. Therefore, when pressed, gyroscopic forces cause a bullet to spin 90 degrees instead of tumble. See this diagram:

enter image description here

  1. The precession does not cause the bullet to point into the direction of movement, but the direction of apparent incoming wind. That means that if there is a crosswind of 20mph, the bullet will turn slightly to point towards the incoming wind while traveling straight forward. See this diagram:

enter image description here

  1. As a bullet drops, there is apparent wind coming from the downward direction. Apparent wind occurs when there is a differential between the object and the surrounding medium. Therefore, the bullet turns down as it drops. See this diagram:

enter image description here

4a) Air resistance forces to the front of the bullet and those to the side of the bullet cause different behavior. Air resistance to the front causes the bullet to reorient into the direction of the incoming force. (See the first image.)

4b)Air resistance to the side of the bullet (i.e., from the left, up, down, or right directions) pushes the bullet to point 90 degrees clockwise to the incoming air. So left-to-right crosswind cause aerodynamic jump down and a or right-to-left crosswind causes aerodynamic jump up. See this diagram:

[enter image description here4

  1. Therefore, intuitively, when a bullet rotates down due to being in a falling trajectory with gravity, it points to the right if it has a right-hand twist:

enter image description here

  1. But every diagram I see of a gyroscope has the gyroscope rotating the opposite direction.

enter image description here

  1. I can’t figure out why the bullet gyroscope rotates in the opposite direction of other gyroscopes. This is not Magnus effect! From Modern Exterior Ballistics, McCoy, “Although the Magnus force acting on a spinning projectile is usually small enough to be neglected, the Magnus moment must always be considered.” Note in the diagram that the Magnus moment points down, and not the to left or right. See these diagrams:

enter image description here

  1. In contrast, spin drift is related to the pitching moment, which does point left or right. See this diagram:

enter image description here

  1. Something to note is that the traditional depiction of bullet rotation may be somewhat inaccurate. (I cannot confirm.) Traditionally bullets are depicted as rotating around the center or mass. Instead, they may rotate around an independent axis. See this diagram:

enter image description here

Thanks so much for the help!

Johnlpmark
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  • About gyroscopic precession: the mathematics to treat that was developed as early as Leonard Euler. That is, ability to treat the problem mathematically has been in circulation for centuries now. However, this has not translated to visceral understanding. What physicists and engineers have to this day is a rule-of-thumb working knowledge, not a visceral understanding. Understanding of bullet angle of repose involves gyroscopic precession and aerodynamic effect. Discussions in books tend to be very very hard to follow because of the lack of visceral understanding of gyroscopic precession. – Cleonis Dec 02 '23 at 07:26
  • I wrote a new answer to the earlier installment of your question. The new answer features a new approach; new information. – Cleonis Dec 02 '23 at 16:22
  • Thanks so much! – Johnlpmark Dec 02 '23 at 23:50

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