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Old February 4, 2024, 04:16 PM   #251
tangolima
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Gas exiting muzzle over takes the bullet momentarily. It is indeed a critical moment, even with perfect crown.

99.9% of a projectile's flight is with a dominant head wind, and therefore it is an indispensable component in stabilization. With head wind suddenly gone and reversed to tail wind, even momentarily, something can easily go wrong.

I have experimented seating bullet backwards. The bullet tumbles every time. Without going into details, a spin stabilized projectile requires center of pressure (CP) be ahead of center of gravity (CG). A tail wind reverses that. The bullet becomes unstable for fraction of a millisecond. Any minute imbalance of air flow, a cross wind, a less than perfect crown, can impart yaw and pitch on the bullet that requires some iterations in bullet's posture in flight to correct.

Not much can be done to avoid this I am afraid. A good crown is of course essential. Some suggested flash cone / can to shield the bullet from cross wind. Its efficacy is questionable unless the cross wind is super strong, such as waist guns on a B17. How about taking away the gas before the bullet exits muzzle? Muzzle device won't work, ported barrel may.

This is for spin stabilized projectile. Situation is more hopeless in fin stabilized airgun pellet. That's why the barrel is rifled even though a pellet does need that for stabilization.

-TL


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Old February 4, 2024, 05:01 PM   #252
davidsog
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Quote:
I’ve seen where the bullet is doing some pitching and yawing before it’s stabilized, some much more than others. Pretty interesting.
It is interesting. The concept of those pitch and yaw movements having an overall effect of zero is weird. It is counterintuitive but if you understand vector math then the vectors cancel each other out. I also think it difficult to grasp how quickly the bullet achieves equilibrium on 3 axis like a glider. Sierra Engineers relate that the stabilization occurs in the heaviest, most powerful bullets in 1/10th of second with most bullets achieving it much quicker. The order of these effects are very small.

Quote:
I have experimented seating bullet backwards.
Current Ballistic math is only approximates behavior in just a few basic shapes. The most common, G2 and G7 refers to just two basic shapes.

https://kestrelmeters.com/pages/g1-g...the-difference

Reversing your bullet is not one of those shapes and represents being a bullet test pilot so I am not surprised if you observe unusual behaviors. In fact, I would think your bullet would tumble and be unstable as you have changed the design relationship of the CP to CG.
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Old February 4, 2024, 06:05 PM   #253
Vinootz
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On page 1 of this thread some were calculating physics formulas. Why not just point and shoot a rifle you’re comfortable with. And if democrats catch wind of that, they may start drafting gun control legislation that requires people to prove they understand physics formulas.
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Old February 4, 2024, 06:32 PM   #254
davidsog
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On page 1 of this thread some were calculating physics formulas. Why not just point and shoot a rifle you’re comfortable with. And if democrats catch wind of that, they may start drafting gun control legislation that requires people to prove they understand physics formulas.


Just use "The Spontaneous Generation Theory" of Ballistics....

https://youtu.be/AngYmfS7pFI?si=K7O_6FWGI2Biqdso
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Old February 11, 2024, 03:04 PM   #255
tangolima
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Another rabbit hole on wind deflection.

Target at 300yd. Wind flag at firing line and every 100yd. The one at 300yd is in scope's FOV so it is handy. The other ones I have to scan with left eye. Wind is varying at different distances. Which flag should I pay most attention to and why?

Say I got 2" wind deflection at 200yd. Same wind up to 200yd, but no wind from 200yd to 300yd. The wind deflection at 300yd is still 2"?

-TL

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Last edited by tangolima; February 11, 2024 at 03:19 PM.
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