Group size and twist rates for the 308 Winchester

7.9 TWIST, RPM AND ACCURACY   Do bullets shoot more accurately with slower twists = lower RPMs; assuming stability of the bullets? Doesn't mean that the other variables don't count; they do. We have some evidence that slower twists work better = better accuracy than faster twists at higher velocities and lower RPMs. First is the 30BR light bullet slow twist for SCORE in modern benchrest. There's a lot on the 30BR and short bullets and slow twists at http://www.6mmbr.com/30BR.html Second is the CBA experimenters shooting higher velocities with slow twists, accurately. See The Fouling Shot, Sept.-Oct. 2008, pg. 195-18. Third is the results of Larry's Gibson's experiments, the results of which suggest that faster twist barrels run out of accuracy earlier than slower twist barrels, as velocity and RPM increase. Fourth is from the Berger Bullets web site at http://www.bergerbullets.com   "Q: What does the recommended twist mean? A: We list a twist rate that will produce the best stability in all situations. Factors like temperature, elevation and muzzle velocity will affect how the bullet will stabilize. The twist rates we recommend will give you the best stability at the slowest twist possible. The recommended twist is based on a range of velocity so it is a guideline not an absolute requirement.   Q: Why do you recommend the slowest twist? A: Spinning a bullet faster than necessary can amplify any inconsistency in the bullet. Since we use J4 jackets you can shoot Berger Bullets in faster twist than what is listed. We list the slowest twist rate needed because we want to squeeze every bit o0f accuracy out of a rifle."   My interpretation of this set of 4 lumps of evidence is that there is a STRONG suggestion that stable bullets shoot more accurately with slower twist barrels at higher velocities = lower RPM. Not proved, proving is hard and expensive. RPM is not the sole, and probably not the most important accuracy determinator. All the other variables, ctg., powder, primer, bullet, alloy, lube, ... affect accuracy. To some degree. To make the suggestion STRONGER, we need more data. But, my interpretation of the data/evidence we do have leads me to the conclusion that I will specify the SLOWEST twist that will stabilize the planned bullet on my next re-barreling of a gun. Because, at up 200 yards, I know of no evidence that a slow twist/stable bullet combination is LESS accurate than faster twists. If Greenhill says that the minimum twist for stability is 1:17", I know of nothing suggesting that 1: <17” is MORE accurate. For a given bullet. That's how I see it.   Pat Iffland: I've said in another thread it's been my experience and a lot of other really good shooters I know that in a thirty caliber starting with a 10 twist at around 1850 fps you can gain about 100 fps more velocity and still maintain BEST accuracy for every inch reduction in twist. Smaller than 30 cal and you can run faster RPM, larger requires slower. Which is why you see 7 and 9 twists in the 22 calibers and 18 and 20 in the 45s. Pat recommended that I look at the Lilja barrel site, this address: http://www.riflebarrels.com/articles/bullets_ballastics/bullet_imbalance_twist.htm>http://www.riflebarrels.com/articles...ancetwist.htm

The crux of the article is in this quote: "It is obvious ”¦ that two aspects of shooting are quite important. Namely, that we use high quality bullets and the slowest twist that will adequately stabilize that bullet.” I suspect that short light bullets are easier to shoot accurately, hence the references to 100-125 grain .30 caliber bullets. Greenhill says that in 30 caliber, a 10” twist will stabilize a bullet 1.42” long, which would weigh ~250 grains. It says that a 15” twist, for example, will stabilize a bullet .95” long weighing ~166 grains. All cast bullets. I think it's clear that 30 caliber twists are generally “faster than needed", that slower twists with shorter lighter bullets are easier to shoot and torque on the bags less. My impression is that gain twist and slow twist barrels are “easier on the bullet” as the bullet accelerates in rotation.