Using the Miller Stability Formula

[Curmudgeon]

I occasionally use this formula to look at bullets I might use. I am trying the new Sierra 110 grain in 6mm. I am trying this bullet in a Dasher with a 7.5 twist. The formula gives me a 1.395 stability factor. This is marginal with a 3% loss of bc according to Berger. Is anyone using a 7.5 twist barrel or have you gone to a 7 twist?
Does bearing surface influence stability or is it only length and weight? Thanks

 

[mikecr]

It's a rule of thumb formula, and should not be taken for more.
Bearing surface is not an input to the formula, so it would do you no good there.
But bearing does matter just as every other individual attribute of a bullet matters to drag and stability. For instance, changing base angle/boat tail end diameter changes drag, which affects stability, which would not show up with the Miller formula, because this change does not affect bullet length or weight(entered parameters).

 

[Berger.Fan222]

I occasionally use this formula to look at bullets I might use. I am trying the new Sierra 110 grain in 6mm. I am trying this bullet in a Dasher with a 7.5 twist. The formula gives me a 1.395 stability factor. This is marginal with a 3% loss of bc according to Berger. Is anyone using a 7.5 twist barrel or have you gone to a 7 twist?
Does bearing surface influence stability or is it only length and weight? Thanks

Sierra typically measures the BC of their bullets in several different twists. The techs there are very helpful and should be able to tell you the twist you need for the full BC.

 

[SheepDog]

As far as I know the Miller formula is the best in existence right now. I am using it in my new ballistics program and have two thresholds for stability. The first is slightly lower twist rate for long range targets and the second is faster twist for hunting.
The Miller formula includes parameters that were not in the other formulas so the Miller formula can correct for pressure, temperature and humidity.

 

[Curmudgeon]

Hello, a rep for Sierra suggested starting .015 off with 32.5 gr Varget. Pretty standard. I was testing to start .010 off with 33 grains Varget. He made the small change as a suggestion. What he didn't say was the bullet won't shoot in a 7.5 twist barrel. If it dries enough by Sunday at range I will test some more.

 

[dcali]

Yes, bearing surface impacts stability - the whole shape of the bullet does. Miller is just a very good approximation, though, and doesn't account for that level of detail. You also have to take those BC loss results with a grain of salt. Bryan made a very good argument that there is can be a small loss below 1.5 Sg, but to my knowledge his testing was limited (someone correct me if I'm wrong). 7.5 ought to shoot well.

 

[Berger.Fan222]

Yes, bearing surface impacts stability - the whole shape of the bullet does. Miller is just a very good approximation, though, and doesn't account for that level of detail. You also have to take those BC loss results with a grain of salt. Bryan made a very good argument that there is can be a small loss below 1.5 Sg, but to my knowledge his testing was limited (someone correct me if I'm wrong). 7.5 ought to shoot well.

Multiple sources (Bob McCoy, Berger, Sierra, Hornady, etc.) report BC decreases as stability is lowered toward 1.0, but there does not seem to be broad agreement on the value of Sg where loss of BC begins as Sg is decreased. Here is a great reference where it is discussed in the Sierra reloading manual:

http://www.exteriorballistics.com/ebexplained/5th/25.cfm

 

[Berger.Fan222]

As far as I know the Miller formula is the best in existence right now. I am using it in my new ballistics program and have two thresholds for stability. The first is slightly lower twist rate for long range targets and the second is faster twist for hunting.
The Miller formula includes parameters that were not in the other formulas so the Miller formula can correct for pressure, temperature and humidity.

Most professional ballistics engineers regard PRODAS as the best way to compute bullet stability. It is an expensive computer modeling program that accounts for all aspects of bullet shape, density, and drag, as well as atmospheric conditions. The Miller formula is likely a close second, and thus becomes the best option in cases where Sg from PRODAS is unavailable.

For shooters who cannot afford PRODAS ($6k), the Hornady 4DOF calculator uses PRODAS for the Sg values it outputs for the bullets it supports:

http://www.hornady.com/ballistics-resource/4dof

I would regard these as more accurate than those from the Miller formula.

 

[dcali]

Don't forget Lapua's new 6DOF calculator, which they oddly opted to release as a phone app. It does gyroscopic *and* dynamic stability. Pretty cool to think of that level of stuff being available to shooters fro free, even if the bullet library is limited.

 

[Berger.Fan222]

Don't forget Lapua's new 6DOF calculator, which they oddly opted to release as a phone app. It does gyroscopic *and* dynamic stability. Pretty cool to think of that level of stuff being available to shooters fro free, even if the bullet library is limited.

Thanks for the reminder. The literature says all the Lapua bullets are supported, which is interesting. Unfortunately, they don't say HOW Sg is calculated, other than saying the method is

"far more sophisticated than the simple Greenhills formula of Miller twist rule."

I hope it is also more sophisticated than the spell check program for their user's manual.

So far, the Lapua app seems too much like a black box. A 6 DOF modeling program is nothing but garbage in-garbage out unless they are using accurately determined aerodynamic coefficients. Likewise the stability calculation method could be something as simple as McGyro or something more reliable like accurately computed (or measured) moments of intertia, etc. in the fundamental gyroscope stability formula. Or it could be a reliable interpolation based on measured Sg values from something like the BRL spark range.

Cool graphs are really only as reliable as the methods used to come up with the values in the graph. Any competent mechanical engineer can integrate 6 equations of motion to get a 6 DOF calculator. Getting accurate aerodynamic parameters is the tricky part.

 

[dcali]

I would assume they calculate it properly with the appropriate aerodynamic coefficients, which as you say, are not a trivial thing to get. It's really a shame they went with a phone app. Such a thing deserves to be on a big enough screen to actually see the charts. But it's still pretty amazing. They give sg vs sd plots that show sd going sideways at range, but the chart is like a 1/4 inch high. That is pretty cool, but come on Lapua!

 

[Curmudgeon]

I appreciate the insight. The Lapua program is still buggy. When it became available I tried using it but use very few of the bullets in their database. The Hornady program has more data in regards to the bullets I use. The JBM calculator still has more input variables. Looks to be a good range day Sunday so I will work more with the 6-110 Sierra bullet. I tend to work more with empirical data since math likes to trip me up. Thanks all.