Gunsmiths generally use the Greenhill Formula to determine the optimal twist for a given bullet. The formula is T=150(d/r) for velocities from about 1500 to 2800 fps. Substitute 180 for the 150 value for velocities exceeding 2800 fps. "T" is the twist rate. "d" is the bullet diameter. "r" is the bullet length to diameter ratio (length of bullet divided by diameter). In .308, it works out fairly well. Sierra 168 grain Matchking is 0.308 inches in diameter. Bullet length is about 1.210 inches so we have a length to diameter ratio of 3.929. Plugging this value into the formula and using the 150 constant (the proven best velocity for the 168 grain MK is 2550 - 2600 fps), we get T=150 x (.308/3.929); T=150 x 0.078; T=11.76 inches or 11 3/4 inch twist …. One rotation of the bullet for every 11.76 inches of barrel traveled. This is very, very close to the "standard" 12 twist barrels on our PSS's and on my Chandler and one of the reasons they shoot the 168s so well. The 175 Matchkings do not perform as well in the 12 twist barrels. Again diameter is 0.308 inches. Bullet length is 1.257 inches, resulting in an ld ratio of 4.081. … plugging into formula (optimal velocity is 2500 to 2550 fps so we will use the 150 constant again) T = 150 x (.308/4.081); T = 150 x 0.0755; T = 11.320. That approximate half inch twist difference makes a big difference in my guns. 12 twist barrels tolerate 175s but do not excel with them. My Hart 26" 10 twist barreled .308 tactical rig, built by John Eckenrode on a Remington 700 SA in a McMillan A2 stock with Ross guard and Leupold 3.5 x 10 M3 LR, eats 168s, 175s, 180s and 190s alive. I'll never go back to slower barrels. The general rule of thumb is to use the fastest twist you can tolerate and match to the longest bullet (usually heaviest) you intend to use for the life of the barrel. I don't remember the exact source of the Greenhill formula. He was a Brit and developed it back in the late 1920s. I obtained the formula from a friend while working at Navy Intel.
While there is a fair amount of "Art" and "Disagreement" in this science of ballistics, I think your "Greenhill" formula is probably very close to being "Right on".
I am shooting a bunch of .308's with 12" twists, three are custom "Select Match" grade, and three are very good factory bbls. My normal procedure with a new gun, is to make a slew of loads starting fairly low, and work up. With 155 Palmas, and 168 Matchkings, it was typical that the groups would all be fairly good, and a few would really shine. I starting shooting the 175 Matchkings this fall, to match the BDC of my Leupold M3 LR, and found an interesting change... at the starting loads the groups were not good for the guns in question... groups were large-ish. But, as the loads got faster, the groups got smaller. To me, this indicated that the twist rate was marginal. At the velocities that I wanted, the groups were within reason for the guns, but I feel that if I drove them faster, they might go tighter.
I'm having a gun barreled by Shilen, and asked them for a 10" twist, and Gary Huntsman of Shilen Said "... Unless you are shooting bullets heavier than 168 grains, I'd use the 12 twist..." which supports the Greenhill formula's results. Also, the M24 bbls have a 11.25" twist, and were designed for the 175gr M118-LR load which uses the 175 Sierra Matchking.
How-some-ever, to throw some confusion into the pot for discussion…
Berger Bullets, says, on their web sight...
.308 CAL 155gr Very Low Drag Recommended twist rate is 1/14
.308 CAL 168gr Very Low Drag Recommended twist rate is 1/13
.308 CAL 175gr Very Low Drag Recommended twist rate is 1/13
.308 CAL 185gr Very Low Drag Recommended twist rate is 1/12
.308 CAL 190gr Very Low Drag Recommended twist rate is 1/12
.308 CAL 210gr Very Low Drag Recommended twist rate is 1/11
… and the Berger VLD's are longer, for a given weight, than the Sierra's???
Would be interested in results (not theories) of shooters that have shot the heavier Sierras and/or Bergers in 12" twist barrels at long range
