Sufficient twist rate ?
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- Somewhere on the Hudson Bay Coast
Maybe, perhaps, and/or sometimes. Bullet stability can't always be qualified with a yes or no answer. For example, if you're shooting in at 4000' elevation on a warm humid day, you might find that your bullet stability holds up. If you take that same load and shoot it at sea level, at -40, you might find that the bullet is as unstable as the individual wanting to go shooting in those conditions.
Furthermore, in a big game hunting scenario, the bullet must not be only stable in flight, it must also have terminal stability, and the terms are not synonymous. For example, when the M-16 was first fielded, the concept was that the bullet would be stable on its way to the target, but tumble in tissue. That is not the performance that you need when sighting on that elk of a lifetime. Bullets precess (yaw) twice in flight, first when they exit the muzzle, and again when they impact a denser than air medium. In either case, the higher the bullet's rotational velocity, the faster it recovers from precession. The faster the bullet recovers from precession, the better it's terminal performance in terms of penetration and wound volume.
As far as stability for punching paper goes, you've heard the term that a bullet sometimes needs to go to sleep before it's accuracy can be fully realized. That's just another way of saying that the bullet needs time to come out of precession, and suggests that it's rotational velocity is marginal. If you examine your bullet holes and see that they are something other than round, the bullet needs to go faster, or be shot from a barrel with a faster twist in order to fully stabilize in the conditions in which you're shooting.
Your best bet is to load up a few of the new Hornady wonder bullets, and determine the stability for yourself. Near range bullet impacts, at 25 yards or even closer will tell part of the story, just as will at 100, 300, and 500 yards. Be sure to test the bullets' stability under the same conditions of elevation and temperature that you intend to use them in. If your bullet has a similar length as the Berger .284/175 XLD, (1.535") Bryan Litz suggests a minimum twist for stability of 1:9.7, and 1:8.7 for optimum performance. Its more fun though, to buy a box and try them so you can see what's going on for yourself.
Furthermore, in a big game hunting scenario, the bullet must not be only stable in flight, it must also have terminal stability, and the terms are not synonymous. For example, when the M-16 was first fielded, the concept was that the bullet would be stable on its way to the target, but tumble in tissue. That is not the performance that you need when sighting on that elk of a lifetime. Bullets precess (yaw) twice in flight, first when they exit the muzzle, and again when they impact a denser than air medium. In either case, the higher the bullet's rotational velocity, the faster it recovers from precession. The faster the bullet recovers from precession, the better it's terminal performance in terms of penetration and wound volume.
As far as stability for punching paper goes, you've heard the term that a bullet sometimes needs to go to sleep before it's accuracy can be fully realized. That's just another way of saying that the bullet needs time to come out of precession, and suggests that it's rotational velocity is marginal. If you examine your bullet holes and see that they are something other than round, the bullet needs to go faster, or be shot from a barrel with a faster twist in order to fully stabilize in the conditions in which you're shooting.
Your best bet is to load up a few of the new Hornady wonder bullets, and determine the stability for yourself. Near range bullet impacts, at 25 yards or even closer will tell part of the story, just as will at 100, 300, and 500 yards. Be sure to test the bullets' stability under the same conditions of elevation and temperature that you intend to use them in. If your bullet has a similar length as the Berger .284/175 XLD, (1.535") Bryan Litz suggests a minimum twist for stability of 1:9.7, and 1:8.7 for optimum performance. Its more fun though, to buy a box and try them so you can see what's going on for yourself.
From the JBM bullet database, it appears that bullet is 1.567" long. Hornady says it has a G7 of 0.347. When you enter that into the Berger Twist Rate Calculator, with 2900 fps velocity, sea level, and 59F temperature you get a stability of 1.19. That is not good. Ideal stability of 1.5 would require a twist rate of 8.5. You could use the calculator to enter your anticipated elevation and temperature to refine it, but I suspect it won't change that much. With that long bullet seated back in the case, it may be hard to get 2900 fps for speed too.
Looking at the Berger Hunting bullets you could shoot a 168 grain VLD Hunting at 2900 and get a stability of 1.5 in a 9.5 twist.
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You need to get these books if you want accurate BC's and what twist rates will work for what bullet .
If you think you are getting accurate BC coefficients from the bullet manufacturers, then have a read of this.
More Inaccurate Specifications of Ballistic Coefficients
Yes, and the document found that Litz's calculations were not always predictive of actual measured BC's. Litz suggested the bullet yaw caused by firing the bullets out of a light sporter barrel were responsible for the measured discrepancy in BC's. The tests do not confirm that theory. Some have suggested the testing was done to make Litz look bad. I think not. It was more of a knock on the whole industry for publishing optimistic BC's. I didn't do a thorough check, but of the BC's that were found to be off, no correction has been made by manufacturers to their published numbers. I recall there was one that was off by 45% and it has not been corrected.
I only bring it up as a reminder that we cannot take these numbers as the gospel. This was the second of two tests done by the same author. I have not been able to find the first document.
I think the Berger Twist Rate Calculator is credible and Litz is probably responsible for the specifics of it. But, it uses BC, and that may have some error...
There is another calculator on line by Kolbe. He was the former owner of Border Barrels. It is credible as well, but requires more bullet dimensional data as a basis of input. Berger is one of the few companies that provides something close to a full disclosure of those dimensions.
Exactly that's why I get them from bryan litz books , don't trust anything on a box of ammo or bullets
Litz don't work for nosler, hornady , lapua, Ect he does work for Berger , he also worked for the Air Force developing and testing missles , so I'd trust his BC's before trusting what the bullet manufacturers post on there box of ammo
- Location
- Somewhere on the Hudson Bay Coast
Ron, whether or not the BCs are inaccurate is irrelevant to this discussion. Stability is dependent upon bullet length, and the bullet's rotational velocity, and BC does not enter the equation, unless the discussion pertains to transonic instability at long range.
As for discrepancies concerning BCs, the results are very different if testing is conducted at high altitude or low, in dry air or humid, in warm temperatures or cold. Consider that Sierra revised their BCs after they moved their plant to Missouri from California. Then there is the likelihood of inconsistencies between chronographs, so its not surprising that BC can be somewhat suggestive. BCs should not be considered absolute, and for this reason the better ballistic programs have a BC correction feature.
BC's will also change with velocity changes. So some people will "step" the BC's in their ballistics program for the appropriate BC's as the projectile losses velocity throughout its flight.
You should look more carefully at the results of the Berger Twist Rate Calculator. When the Stability Factor drops below 1.5, the calculator estimates a degredation to the BC. In other words if you do not have enough spin, the stability is still OK, but the Ballistic Coefficient suffers, and down range velocity suffers. The BC calculations become inaccurate. Where this becomes relevant is when you do not have enough twist to get a 1.5 stability factor. In some cases a higher BC bullet even after degredation due to reduced stability may be the higher BC, and still worth considering.
Bullet length is the biggest factor in determining the stability factor, but not the only one. The actual bullet shape, and density is relevant too.
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That is typically done when the G1 coefficient is used instead of the G7. The G7 is much more predictive of long range performance, and does not need the stepped changes. Sierra uses the stepped G1 approach, which is a little behind the times, and they don't report G7. I see Hornady, at least on some bullets are now providing a G7.
Read the books applied ballistics , you will like them books
- Location
- Somewhere on the Hudson Bay Coast
A point to consider is that any change in shape must also change length if there is no change in weight, material, or caliber. Add a boat-tail, the bullet is longer, a secant ogive is longer than a tangent, a shorter bearing surface equals a longer bullet, etc, etc. Likewise bullet density directly impacts length which becomes obvious if monometal bullets and lead core bullets of equal weight are compared. As for intentionally choosing a bullet which has a stability factor below 1.5, that might work under ideal conditions, but use it on a cold, dry, day, at sea level, and you'll be disappointed. Then again I like fast twist barrels, so I'm normally well above 1.5.
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- Location
- Somewhere on the Hudson Bay Coast
I use Applied Ballistics phone app, and Accuracy 1st's Whiz Wheel.
I know who Litz is. I've exchanged posts with him on another form on twist rate calculations. I never said he worked for anyone but Berger. Also have exchanged PM's with Don Miller who is the originator of the frequently used Miller Formula for twist rate calculations.
