40 gr. nos ballistic tip stability

[boltman223]

I was messing with bergers twist rate calculator and put in the info from my 221 fireball. It gives a SG of .885. This is with nosler b-tips, they measure .703". The barrel is a 14" twist. I have them running at 3250 and im at 900 feet elevation. The accuracy is great, 3 shot groups in the .2's and .3's. No key holes or anything.

Berger says .885 is unstable and expect bad accuracy, tumbling bullets and key holes.

Am I inputting something wrong? Or am I missing something? Any info would be great.

I have been kicking around the idea of building another 221 and am trying to decide between a 14" or 12" twist.

 

[boltman223]

I know benchrest shooters have really slow twist but, is their SG this low?

 

[CatShooter]

I was messing with bergers twist rate calculator and put in the info from my 221 fireball. It gives a SG of .885. This is with nosler b-tips, they measure .703". The barrel is a 14" twist. I have them running at 3250 and im at 900 feet elevation. The accuracy is great, 3 shot groups in the .2's and .3's. No key holes or anything.

Berger says .885 is unstable and expect bad accuracy, tumbling bullets and key holes.

Am I inputting something wrong? Or am I missing something? Any info would be great.

I have been kicking around the idea of building another 221 and am trying to decide between a 14" or 12" twist.

Their twist rate calculator is defective (i.e., broken).

I have run many known and proven bullet/twist/velocity combinations through it and almost all of them come up as unstable

 

[Gabe22BR]

Hornady has a calc that I use, it's better than bergers. I shoot Sierra's 50 gr. BK in my fireball.

 

[dcali]

Every stability calculator on the market for sporting bullets is approximate. Calculating stability with more precision requires data that generally just isn't available.

That said, if you're running on the ragged edge like that, don't be surprised if a change in conditions pushes you into an unstable region.

But the proof is on the target, so carry on.

 

[CatShooter]

Every stability calculator on the market for sporting bullets is approximate. Calculating stability with more precision requires data that generally just isn't available.

That said, if you're running on the ragged edge like that, don't be surprised if a change in conditions pushes you into an unstable region.

But the proof is on the target, so carry on.

The stability algorithm that is included with the Sierra Infinity Suite V-7 is G-R-E-A-T...

I have done many "What-ifs" with it, and it matches every popular and empirically known bullet/velocity/twist combination/recomendation that are used in the shooting community. It is "Plug 'n' play".

Some of the other bullet maker's programs do too. It is Berger that is broken, not the whole class of software algorithm.

There are other problems with the Berger programs too.

I know that some think if it has the name Berger on it, it is better that everything ever made, but I do not think that their ballistic algorithms were well written.

 

[hogpatrol]

What does it do at 200 & 300, five shots?

 

[boltman223]

What does it do at 200 & 300, five shots?

It shoots great at 200 and 300. I don't think I've ever had a 3 shoot group over 2" at 300 most around 1.5" and less. 5 shot groups at 100 are in the .5 range and under. I can't remember what five shot groups are like at 300 but, I remember I was happy.

Ive zeroed this gun in when it was in the 20's F and there was no signs of instability. The groups were bigger, I can't recall size exactly but, exceptible. I think that should be attributed to temperature though

I'm know there are lots of people having good luck shooting 40 and 50 grain bullets in a 14" twist. Remington sells their 50 gr. Accutip for the 221 and I don't think they would sell something that is most likely going to keyhole. Even the 222 with a 40 nos b-tip shows unstable at 3600 fps until 4000' elevation.

All in all I'm just surprised of the SG with the results I have had.

 

[Bryan Litz Ballistics]

The Berger stability program runs the Miller stability formula which is a well known and accepted formula for predicting stability for modern bullets. This formula is most accurate for most modern bullets having relatively long noses and boat tails. Bullets with short noses and flat bases (or very short boat tails) will have their stability 'under-predicted' by the Miller formula, as you've seen with the subject bullet.

As damoncali stated, stability predictions are approximations. The old Greenhill formula was good for the short nose, flat base BPCR bullets and lower MV's that were prevalent when that formula was established. That (Greenhill) formula is not accurate for modern bullets which are longer, pointier, have BT's and get fired at higher velocity. If the Sierra program is producing more accurate stability predictions for varmint bullets, my guess is that it's running greenhill (or a variant) and would be less accurate for modern pointier BT bullets. But I really don't know what it's running.

The JBM stability calculator runs the MC Gyro program which is more accurate for a wider range of bullets, but requires a long list of bullet dimensions and is more complicated/harder to use. (http://www.jbmballistics.com/cgi-bin/jbmdrag-5.1.cgi) We decided to implement the Miller option that is less complicated and prone to user error for the Berger calculator, but it does compromise on prediction accuracy for the flat base bullets. On the plus side, the inaccuracy is always conservative, meaning it errs on the side of recommending faster twist so it will never recommend a twist that's too slow.

-Bryan

 

[CatShooter]

If the Sierra program is producing more accurate stability predictions for varmint bullets, my guess is that it's running greenhill (or a variant) and would be less accurate for modern pointier BT bullets. But I really don't know what it's running.

-Bryan

The sierra program in NOT the greenhill program... it was devloped by two guys that did ballistics for the NASA aerospace program - it works with long, boat tail bullet just fine.

 

[Bryan Litz Ballistics]

What inputs does the Sierra program require to calculate stability?

 

[boltman223]

Thank you for the clarification Brian.

 

[dcali]

What inputs does the Sierra program require to calculate stability?

https://www.sierrabullets.com/documents/Infinity7.2.1.UserGuide.pdf

Page 42.

"The practical analytical model for rifling twist rate, attributed to William F. Davis in Infinity 6, is based on practical
assumptions and then verified by practical experience in the field."

In other words, it's a rough but decent approximation, like every other calculator out there aimed at sporting use. Given the inputs required, it is impossible to be anything else.

 

[JPeelen]

Damoncali,

many years ago the late William C. Davis published some of his ballistics programs in the American Rifleman, and I bought them from him/Tioga Engineering on his BALTEC1 diskette.
His bullet stability program is called GRNHIL.BAS and uses Greenhill's formula. His modification consists of re-computing "Greenhill's constant" based on length, diameter, and twist rate instead of using a constant value of 150.

W.C. Davis did us shooters an extremely great service by making his programs available. At the time there was nothing better available. But my opinion is he would have embraced the late David Miller's formula. I know at least one other professional ballistician who basically agrees with Bryan Litz that Miller's formula is good for modern slender, boattailed bullets, and underestimates the stability of square based bullets.

Having used Infinity 5 and 6, which used very old techniques, I do not interpret the text for Infinity 7 as pointing to a totally new formula. I they had a new formula, why didn't they publish it like Davis and Miller did?