Miller Formula

[1shot]

I'm interested in opinions/practices of shooters that make a decision on buying a specific bullet and or rate of twist based on the Miller formula. The "rule of thumb" is not less than 1.4Sg, and as stated, “some accuracy can be gained by keeping it under 2.0”. Can someone tell me how much "wiggle room” you allow yourself to have on either end of the set parameters?
Thanks,
Lloyd

 

[Michael Courtney]

There is not much wiggle room at all below Sg = 1.4. Your bullets might not tumble at Sg = 1.2, but there will be a significant increase in drag, and worse yet, significant shot-to-shot variations in drag. However, keep in mind that we've published an improved twist rule for plastic tipped bullets, as the original Miller twist formula significantly underestimates the required twist for plastic tipped bullets. Spinning too fast (Sg > 2) is more of an issue with less precise bullets where there is more physical separation between the geometric center and the center of mass. Spinning too fast is also more likely to be an issue with a thin sporter barrel than with a heavy match barrel.

 

[jo191145]

Your average 6PPC runs a 14 twist barrel with 65-68 grain bullets.
Run that through any of the stability programs and see what you come up with.

Mine shoots dots at 1.018 SG Just sayin.

 

[Michael Courtney]

Your average 6PPC runs a 14 twist barrel with 65-68 grain bullets.
Run that through any of the stability programs and see what you come up with.

Mine shoots dots at 1.018 SG Just sayin.

A few of points:

1. An increase in BC is more expected at Sg < 1.3 than a decrease in accuracy. Have you measured the BC?
2. A good match bullet in a good barrel can often tolerate being shot at Sg < 1.3.
3. The ambient conditions are important for an accurate computation of Sg. An Sg of 1.018 at sea level (P = 29.92) can be much higher at higher elevations. I can shoot 69 grain match bullets in a 1 in 12" twist .223 Rem at Co Springs area ranges because of the elevation.
4. The Miller rule assumes that the lead in the nose of the bullet extends all the way to the hollow point in match bullets. If there is a significant empty space in the bullet nose, the actual stability will be higher than predicted by the Miller formula. I use a paper clip or the lead from a mechanical pencil to measure the depth of the air space in the nose of a bullet. A lot of hollow point match type bullets have a signficant empty space in the nose.

 

[Michael Courtney]

Double post. Sorry.

 

[MVW]

Brian Litz's book on External Ballistics for Long Rnage Shooting has an extensive discussion on stability and Miller's formula. It doesn't necessarily disagree with anything stated here, it is just layed out in a more complete discussion than is available in a forum setting. It is well worth the read.

Mike

 

[1shot]

Please let this thread continue as I’m learning more every day. Thank you all for your input as you made me realize that I ignored the obvious. My bullet of choice was far longer than advertised and accordingly it took me from 2.16Sg to 1.83Sg. Along with a change in seating dimensions this cut my extreme spread by over 66%. The groups are stellar at 100 yds (.204). But I’ll report back tomorrow how they translate at 1,000.
Thanks again,
Lloyd

 

[jo191145]

Michael, point #4 of your post may go a long way to explaining it.
Thanks, never heard that before. Admittedly I care very little what the formulas say. I'm not designing new bullets so have found little use for it but for those that find the need it might be very relevant.

 

[Michael Courtney]

Brian Litz's book on External Ballistics for Long Rnage Shooting has an extensive discussion on stability and Miller's formula. It doesn't necessarily disagree with anything stated here, it is just layed out in a more complete discussion than is available in a forum setting. It is well worth the read.

Mike

The Litz book is highly recommended.

Don Miller's articles on the original Miller twist rule are also recommended:

http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf

I would also recommend the two stability articles that I co-authored with Don Miller from the Jan 2012 and Feb 2012 issues of Precision Shooting.

 

[Michael Courtney]

Michael, point #4 of your post may go a long way to explaining it.
Thanks, never heard that before. Admittedly I care very little what the formulas say. I'm not designing new bullets so have found little use for it but for those that find the need it might be very relevant.

Yes, this was a point that concerned Don a great deal and that he felt had gone underappreciated. Before he passed away, Don and I had brainstormed about some experimental and theoretical approaches to resolving the issue (accounting for it in a new formula), but we did not complete the work. I do have some bullets loaded and more experiments planned when time and circumstances allow.