Ballistic Coefficient

[dcali]

I think shooters tend to take Bryan’s advice a little out of context. The 1.5 SG provides a very small improvement, and I would be surprised if it’s not a bullet-specific improvement. If you’re shooting expensive high BC bullets and trying to wring out the last percent of BC, then sure, spin them up *a little*. That’s according to the limited test in the book at least. Perhaps he’s done more on this that I’m not aware of. Maybe there is some benefit to dynamic stability at very long range- I have to plead ignorance on that.

Depending on the game being played (and the conditions you happen to be facing), that might not be a good tradeoff with the benefits of a slower twist that we have known for years. (Increased precision, decreased aerodynamic jump, reduced torque, and decreased bullet stress). With mid range f class matches being won by 600-40x scores, do you really want to add 10-15% to your group size with a super fast twist just to gain a percent or two in BC? You definitely don’t want any part of that in short range benchrest, and probably not in long range benchrest. It might be worth it in ELR, or 1000 yard f class where wind plays a bigger role. You have to run the numbers to see.

 

[Webster]

The real question is whether someone is actually shooting a sufficient distance that the small effect provided by these extreme twist rates really is a benefit. If not, there are likely more downsides to using them, including reduced precision and gun handling issues due to increased torque.

I have actually been leaning the other direction recently. Litz used to recommend a gyroscopic stability coefficient (Sg) of at least 1.4. Later he increased the minimum recommended Sg to 1.5. According to these suggestions, if you aren't generating an Sg of 1.5 (or greater), the bullet is not achieving its maximum intrinsic BC. For some time, I have been using the Berger 90 VLD and and 200.20X bullets out of 7.0-twist, and 11.0-twist barrels, respectively. Both of these twist rates are insufficient to produce Sgs of 1.5 with the two respective bullets at the velocities I achieve in .223 Rem and .308 Win cases.

However, LabRadar velocity data suggests they are achieving BC values very close, if not identical, to those predicted on the box for unpointed bullets, or with a predicted 4-6% BC increase from the box value for pointed bullets. I understand that the LabRadar is not the preferred instrument for calculating BC, primarily due to its limited range for measuring velocity drops. Nonetheless, the LabRadar data suggests that these bullets are not giving up a significant amount of BC relative to the published values, even when fired with Sg values closer to 1.4, rather than 1.5. I suspect any variance or limitations of the values I have been obtaining using the LabRadar data are small enough that they're not worth talking about. I certainly can't shoot the difference. So in my hands, running bullets at twist rates predicted to provide Sgs around 1.4 appears to provide some advantages, including obtaining very close to the box value BC, along with less recoil and lower potential for jacket failure.

The OP was talking about a hunting rifle out to 350 meters.

 

[watercam]

The real question you should be asking in a hunting scenario is when will my bullet not expand. Usually around 1800-2000fps.
Something I rarely hear so called long range hunters talk about. If you shoot a high BC bullet that you can extend your range a bit as they hold higher velocity a bit longer.

 

[Ned Ludd]

The OP was talking about a hunting rifle out to 350 meters.

Actually, that was a hypothetical scenario proposed by the OP as the kind of information he would like to see on a box of bullets. Regardless, I was responding directly to Laurie's post about faster twist rates and their effect on BC just above mine.