Berger Stability Calculator doubtful result?

[Jelen]

What's going on?

I've just input the data for a Nosler 40gr B/Tip in .224. From Nosler website: Bullet length 0.701". BC 0.221.
Inputting a 1 in 12 twist, mv3300fps, Temp 70F
Result: Marginal Stability SG 1.2. Recommends a twist of 1in10. ?????
Trying a B/Tip of 50gr was even worse.

Kind answers on a postcard please.

 

[Will the chippy]

I've tried that too for my 1 in 14 22250. 50grain is not stable apparently.

 

[Laurie]

The Miller Rules formula (which the Berger calculator uses) is geared to conventional FMJBT, HPBT or BTSP form bullets. There are two features on varmint type bullets that can (and usually do) give overly low Sg values. First, many such bullets barely have a boat-tail, only a slight bevel. Miller / Berger doesn't work for flat-bases and the actual twist rate needed is in the order of +1 to +2 in terms of inches barrel rifling twist compared to a same length boat-tail design. (That's why the Berger calculator has a link to a table of recommended twist rates for its flat-base designs.) I imagine many light varmint bullets fall between true match style boat-tail and FB shapes and required twists with it, but can't comment on the Nosler.

Secondly, and likely having a much greater effect, is the synthetic bullet tip which increases the OAL considerably, but is made of a much lower density material than the rest of the bullet. Miller used a compromise density value for the mix of gilding metal (brass) and lead in conventional lead-core bullets. The precise effects of a low density tip increasing the bullet OAL is a bit contentious, but it's generally accepted that as with FB designs you need a lower rotation / slower rifling twist than for a conventional BT bullet.

This (plastic tipped bullet stability) issue has been looked at in other posts on SD. Here's a fairly recent thread on it with links to the Miller-Courtney calculator that takes this factor into account.

Bullet stability

 

[Jelen]

The Miller Rules formula (which the Berger calculator uses) is geared to conventional FMJBT, HPBT or BTSP form bullets. There are two features on varmint type bullets that can (and usually do) give overly low Sg values. First, many such bullets barely have a boat-tail, only a slight bevel. Miller / Berger doesn't work for flat-bases and the actual twist rate needed is in the order of +1 to +2 in terms of inches barrel rifling twist compared to a same length boat-tail design. (That's why the Berger calculator has a link to a table of recommended twist rates for its flat-base designs.) I imagine many light varmint bullets fall between true match style boat-tail and FB shapes and required twists with it, but can't comment on the Nosler.

Secondly, and likely having a much greater effect, is the synthetic bullet tip which increases the OAL considerably, but is made of a much lower density material than the rest of the bullet. Miller used a compromise density value for the mix of gilding metal (brass) and lead in conventional lead-core bullets. The precise effects of a low density tip increasing the bullet OAL is a bit contentious, but it's generally accepted that as with FB designs you need a lower rotation / slower rifling twist than for a conventional BT bullet.

This (plastic tipped bullet stability) issue has been looked at in other posts on SD. Here's a fairly recent thread on it with links to the Miller-Courtney calculator that takes this factor into account.

Bullet stability

Thank you Laurie. Since posting i've re-discovered JBM and that gives SF's that support the 50 -55gr bullets that 90% of shooters seem to be using in 1/12 twist rifles. Basically a bullet with an SF of >1.4 will do the job for my needs.

 

[Laurie]

Basically a bullet with an SF of >1.4 will do the job for my needs.

1.4 was Miller's advised value for many, many years. It was increased to 1.5 (as used in the Miller / Litz / Berger calculator) after Bryan Litz discovered in long-range field testing that lower-than-1.5 values increased drag / reduced BC. This is shown in the Berger calculator results, but looking at the calculated reductions for 1.5 down to 1.4, they are modest even for bullets designed for long-range match applications. Taking the Berger 7mm 184gn LR Hybrid - state of the art in long-range F-Class, BR and suchlike in the 7mmWSM or 7/6.5PRC, the calculated G7 BC drops from an average of 0.356 to 0.344 at 2,900 fps MV, a tad over 3% between a 9-twist rifling pitch (Sg 1.39) and the optimal 8.6 rate (1.52). I suspect that the real values / difference vary by flight distance too, so see a greater real BC reduction at 1,200 yards than 200.

For the serious F-Class shooter (say) with state of the art rifle, optics, SEB rest etc buying 184 Bergers at over £1 each, optimising everything makes sense (assuming he or she can get the rifle bits, supply of top-rate powders, bullets etc, not to mention afford them). Given the outlays of time, money and everything else to reach the pinnacle of competition results ensuring the barrel has an optimal or faster twist rate is a given. For the rest of us as long as it shoots well enough in whatever application ............... ?