Interesting bullet holes - Explain this?

[Laurie]

Sierra designed a series of relatively short and blunt nose semi-pointed flat-base bullets (Sierra code: SMP) specifically for the 14-twist 222 Rem. Some, maybe all, are still listed. These go up to 60gn and they work fine in the 222.

As @dromia says, bullet weight is a red herring in this discussion and can be very misleading in some cases where assumptions can be 180-degrees 'out'. If you have two bullets of identical length and form in a calibre, but of two different weights, the heavier model actually needs a slower twist than the lighter model. This is one reason why all-copper 'monolithic' bullets cause all sorts of stability issues. Not only are they longer than an equivalent lead-cored design, but are also lighter.

Note for those who've downloaded the Millers Rules bullet stability spreadsheet, it is only accurate for 1) copper jacketed lead cored models; 2) boat-tail designs.

Flat-base bullets manage with a slower rotation / higher twist rate than equivalent length BT models. As a rule of thumb, if Miller (plus JBM ballistic apps or whatever) says you need a turn in x" for stability, a same length flat-base is fine on x + 1" twist, sometimes even a bit more. Berger gives a fine practical example on its bullet stability online facility:

https://bergerbullets.com/twist-rate-calculator/

which has an internal link to a sub-section:

https://bergerbullets.com/flat-based-bu ... ist-rates/

Using the Berger calculator, Miller or JBM, the 88gn HBC FB Varmint 0.243" bullet which has an OAL of 1.076" needs 1 turn in 9" twist to achieve the optimal 1.5 Sg value, but Berger actually advises 10" - very handy for 243 Win shooters.

[Mattnall]

Twist rates and bullet stability relate to a bullets length not its weight, unfortunately it has become common practice to use a bullets weight as a proxy for its length which is very unhelpful in discussing the subject and only complicates the issue and confuses those shallow of knowledge on the subject.

The Miller Formula does incorporate bullet weight along with length and is meant to give a better calculated twist rate than Greenhill's.

[daman]

Twist rates and bullet stability relate to a bullets length not its weight, unfortunately it has become common practice to use a bullets weight as a proxy for its length which is very unhelpful in discussing the subject and only complicates the issue and confuses those shallow of knowledge on the subject.

The Miller Formula does incorporate bullet weight along with length and is meant to give a better calculated twist rate than Greenhill's.

The Berger calculator uses ballistic coefficient, calibre, weight and length. https://bergerbullets.com/twist-rate-calculator/
But with only a shallow knowledge, the weight has always worked for me as a reasonable catchall. Perhaps when I'm older and wiser I'll care more :-)

[Mattnall]

But with only a shallow knowledge, the weight has always worked for me as a reasonable catchall. Perhaps when I'm older and wiser I'll care more :-)

Generally weight is a good indicator as heavier bullets are longer, usually. Unfortunately it doesn't work for long light bullets like tracer, for a 62gn 5.56 tracer you need something like a twist rate that would just stabilise a 77/80gn bullet.

[daman]

Thanks Mattnall (and Laurie and Dromia).

I don't (usually) mean to be flippant, and I do appreciate the depth of wisdom and accumulated knowledge on this site. As a decidedly 'average' shooter who does not load my own or shoot exotic things, I'm always glad when those with more experience/knowledge share.

-daman