Thread: SS109 load data
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Old February 1, 2020, 05:43 PM   #16
Join Date: March 4, 2005
Location: Ohio
Posts: 17,188
The off-center CG circles around the bore axis as the bullet goes down the tube. Think of it like a weight you are spinning on a string. The instance the muzzle releases the bullet, like letting go of the string, the bullet stars moving in a direction tangent to the side of the bore it was closest to and in the direction of the barrel twist. That fling is slow (usually on the order of inches to a couple of feet per second), but stays with the bullet all the way to the target, drifting to the side (or up or down or in any direction in between, depending on where the CG was when it cleared the muzzle). It lands at a distance from the POI that is the time of flight times the drift velocity it introduces.

The wobble is also there and decreases the BC some by adding to drag a bit.

A horizontal torsion pendulum rests the bullet in a small trough and you can rotate it in the trough to see if the angle of suspension changes. Very slow process.

A bullet spinner does spin the bullet, as you describe. There are different designs out there. One created by Harold Vaughn uses compressed air to spin the bullet in a small tube and a vibration pickup to detect wobble. One created by Mid Tompkins that Bart B. has described before was a special collet made for a Dremel tool operated through an ammeter. The tool bullet was mounted in the collet and the tool turned on and if the bullet was unbalanced the motor's current demand would increase because of the added force needed to spin despite the axis wobbling (fighting the gyroscopic stabilizing tendency of the tool's own spin, plus some increase in bearing friction that would be associated with that).
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