I tried to log on to Bryan Litz website, but the link is down. Below a quote from German Salazar's blog, which, I think paraphrases Litz:
If a bullet's Sg is between 1.0 and 1.4, the bullet should be stable enough not to tumble, but it won't necessarily settle into a stable flight. This marginal level of gyroscopic stability means that the initial pitch and yaw motion that every bullet has won't damp out; consequently, the bullet will have a lower effective BC because of the higher drag induced by the pitching and yawing motions and accuracy will be compromised. These problems will increase as the bullet travels downrange, so unlike the bullet with adequate gyroscopic stability (Sg at least 1.4) which gets more stable, the marginally stabilized bullet will become less stable as it travels downrange.
OK, downrange performance of a bullet isn't too important in PB benchrest. Historically, 14-twist barrels performed very well. What, if anything, has changed? Lets' play the "what if" game:
1. Maybe the yardstick has changed. Aggs are getting better. What use to be "very good" is now only "acceptable."
2. Bullets have changed. Randy says the extra length is worth .5 increase in twist, and a boattail another .5.
[random graspings: Somebody mentioned the effect of a "dual radius bearing surface," and Vaughn addressed that in his book, though not as a factor in twist rate, rather in affecting in-bore yaw. From a simple-minded point of view, a dual-radius shank itself should not affect Sg -- the form of the bullet remains the same. But the observed effect -- greater in-bore yaw -- increases the tip-off rate, and that marginally affects pitch & yaw -- "lowers ballistic coefficient." Does that mean a slight reduction in Sg?]
It gets very hard to predict what will happen in any marginal situation when you tip the scales just a little bit more.
[left field thought: Point-blank benchrest has also taken jamming the bullet as a revealed truth. All of a sudden, a few people are jumping certain bullets. I can think of several theoretical pluses and minuses to either jam or jump. For example: if you're running a tight neck AND you're loading concentric ammunition, the self-centering advantage of jamming a bullet is less important. If the powder you use doesn't "prefer" high neck tension, the effect of jamming a bullet (equivalent to more neck tension) is less a factor. Etc.]
Well, it is winter on BR Central. All the above is tinkering with "what ifs" using the theoretical model. Let's get practical. I'll buy a 12-twist barrel. But I don't shoot PB benchrest well enough, often enough, to get reliable test results. What we need is a National-level shooter. One who is having enough issues with his/her equipment to be willing to try something new.
Conclusion:
Jackie, I'll pay for a 12-twist 6mm barrel blank if you'll chamber it and test it out. Somebody else willing to contribute towards other supplies he'd need?
Edit:
I realize full well that one barrel proves little. From one point of view, "it's just as good" would be very encouraging news. The only thing in it for Jackie is it might be a very good barrel . . . and that's not much. But he's proven over and over what an asset he is to our sport, so maybe we can go to the well another time.
) show instability. And others that shouldn't work... hold together and win.
