And here's
another calculator that does it by bullet data with more detail. This will show you what is optimal.
The calculator above is based on Robert L. McCoy's old McGyro program. The JBM site uses a simplified estimate worked out by one of McCoy's student and coworker, Don Miller. Neither is dead on every time, but usually agree pretty well.
The most basic difference is the Miller estimator give you gyroscopic stability factor, while the McGyro-based calculator assumes you want a twist that gives you a gyroscopic stability factor of 1.5, and gives you a graph of what that twist is, verses different muzzle velocities.
Sierra told me long ago that they recommend a gyroscopic stability factor or 1.4 to 1.7 for maximum match accuracy, and 1.3 to 3.0 for "hunting accuracy". 1.5 is probably the most commonly cited as "optimal", though Harold Vaughn liked 1.4 better, and one authority (I am failing to recall which one) liked 1.7 best. But frankly, I've seen darn good accuracy from Sierra .308" 168 grain MatchKings all the way up to about 2.5. The common service rifle 10" twist, originally intended for 220 grain round nose .30-40 Krag ammunition, is too fast to be optimum with any of the more common 150, to 180 grain weights, and yet people swear even stubbier 125 grain bullets shoot well from them. This is why the SAAMI standard barrel in .308 Winchester has a 12" twist. The 11" twist was popularized by, I think, the Navy Marksmanship Unit originally, and that would be for the long 173 grain M1 Type match FMJ bullet used in M118SB.
The reason to avoid spinning too fast is every little bit of mass asymmetry starts to contribute wobble to the bullet's flight, plus you can cause core stripping, where the core slips inside the jacket, if you apply too much rotational acceleration. Either opens groups up. Too slow a spin opens them up, too because the bullet can't settle yaw out quickly enough to stay as cleanly on path. I take that 1.3 lower limit seriously. I've never had a bullet turning at below 1.3 that shot a tight group.
