01-09-2022, 10:36 PM
GF,
My understanding is that there is a range of twist within which a bullet will be stable. Too much spin and the bullet resists pointing anywhere other than the angle of the barrel when it exited. Too little and like a spinning top it wobbles according to any imperfections in the bullet. JBM Ballistics has a formula developed by Don Miller (the 'Miller' formula) that we can use. Apparently it is better than the older Greenhills formula, not that I have ever come across it.
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
With Miller, you enter twist, velocity, weight, bullet length, type of bullet etc, and the algorithm spits out a number. Between 1.3 and 2 is considered stable, so you can see your bullet approaching unstable as the velocity drops, or becoming too stable as you choose faster twist barrels.
Berger Bullets also has an online calculator with a similar system - it might even be the same formula.
https://bergerbullets.com/twist-rate-calculator/
The 223 and 6.5 match rifles I have have different barrels that screw in and out. I have tried different twists in both calibres over the years but am always left wondering whether group size is down to twist, or just that the barrel was a hummer or a dud. Plenty of discussion on the forums with F/TR guys talking about what twist to use for what bullet but I don't think it is as simple as going a heavier bullet for its higher BC and going a faster twist. All things being equal, heavier bullets come out slower and stability is based on velocity as well. Your bullet comes out stable but as it slows down, downrange it becomes unstable at some point. Hopefully not before it hits the target.
So, entering muzzle velocity into these calculators is one thing - entering the predicted velocity at the target will be a different story. My experience of F/TR is that group size is not a neat cone, getting predictably bigger as it goes down range, it's more like the shape of a trumpet. The further the distance is between apogee and target the more issues an unstable or too-stable bullet will impart.
My understanding is that there is a range of twist within which a bullet will be stable. Too much spin and the bullet resists pointing anywhere other than the angle of the barrel when it exited. Too little and like a spinning top it wobbles according to any imperfections in the bullet. JBM Ballistics has a formula developed by Don Miller (the 'Miller' formula) that we can use. Apparently it is better than the older Greenhills formula, not that I have ever come across it.
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
With Miller, you enter twist, velocity, weight, bullet length, type of bullet etc, and the algorithm spits out a number. Between 1.3 and 2 is considered stable, so you can see your bullet approaching unstable as the velocity drops, or becoming too stable as you choose faster twist barrels.
Berger Bullets also has an online calculator with a similar system - it might even be the same formula.
https://bergerbullets.com/twist-rate-calculator/
The 223 and 6.5 match rifles I have have different barrels that screw in and out. I have tried different twists in both calibres over the years but am always left wondering whether group size is down to twist, or just that the barrel was a hummer or a dud. Plenty of discussion on the forums with F/TR guys talking about what twist to use for what bullet but I don't think it is as simple as going a heavier bullet for its higher BC and going a faster twist. All things being equal, heavier bullets come out slower and stability is based on velocity as well. Your bullet comes out stable but as it slows down, downrange it becomes unstable at some point. Hopefully not before it hits the target.
So, entering muzzle velocity into these calculators is one thing - entering the predicted velocity at the target will be a different story. My experience of F/TR is that group size is not a neat cone, getting predictably bigger as it goes down range, it's more like the shape of a trumpet. The further the distance is between apogee and target the more issues an unstable or too-stable bullet will impart.