I'd go further than CatShooter and say Greenhill is misleading not to say downright inaccurate with regard to most modern bullets. It was designed for lead bullets in .45" and larger calibres at very low MVs by today's standards.
The formula that most ballisticians use nowadays is the
Miller Twist Rule and it's available free as a little ExCel program. Just Google the three words and you'll find it elsewhere on Accurateshooter to download. You do need to know the length of the bullet though. A more advanced program is available on the Border Barrels website
http://www.border-barrels.com/barrel_twist.htm
but it requires nose, boat-tail lengths as well as overall length, plus meplat diameter.
Miller and others including Bryan Litz (
Applied Ballistics for Long Range Shooting - a book you
must get a copy of if you're interested in this sort of thing) recommend a minimum stability factor (Sg) of 1.4 in order to deal with adverse atmospheric circumstances, although the theoretical minimum Sg value needed is 1.0.
This raises the question of what is an 'ideal' rate and Sg value for any particular bullet + barrel / rifle / shooter's requirements? Traditionally bench rest competitors and British
Target Rifle shooters, the latter using standard and often poor quality milspec 7.62X51mm ammunition until recently, went for the minimum or slowest twist that stabilised the bullet to the maximum range to be shot over. This was because any unnecessary extra rotational velocity would exacerbate any out of balance factors in the bullet such as the jacket being thicker on one side than the other that cause the bullet to veer off its intended trajectory. Ballisticians increasingly advise us to go for a faster twist these days than is strictly necessary as current commercial match grade bullet quality is very high indeed.
Then there is the issue of ambient temperature and air pressure. A bullet added to twist rate that is optimal at Raton NM 6,000 odd ft ASL in high summer for a particular bullet at a particular MV, may fail to stabilise it at sea level, in high-pressure anti-cyclonic conditions at -20 degrees C in mid winter Canada or Norway say. As a result, Bryan Litz lists three stability factor values for every bullet listed in his book's data pages against a range of twist rates. They are:
Best case - 1,000 ft ASL, 100% humidity, 100 deg F, 3,000 fps MV
Nominal - sea level, 50% humidity, 59 deg F, 2,800 fps MV
Worst case - sea level, 0% humidity, 0 deg F, 2,600 fps MV
If you take a 1-9" twist 0.224" cal barrel, one of the best fits is the 73gn Berger Target BT which has Sg values (Miller Twist Rule generated) of 1.62, 1.41, and 1.22 for the three cases listed above.
The 69gn Sierra MK, a bullet I would say is tailor-made for 1-9" twist 20" barrel .223s is 1.90, 1.66, and 1.43 for the three cases, in theory a bit 'over-stabilised', in practice not at all for any practical use of the cartridge.
The 73gn Berger is 0.970" long; the 69gn SMK 0.900", considerably longer than your 0.832" which would limit you to lighter non-optimal bullets BC- wise, although the definition of 'optimal' in this regard depends upon the distances you intend to shoot over obviously.
Here is how the Miller Formula says a bullet of this length weighing 60gn would look at 2,800 fps MV in 'standard' atmospheric conditions:
Don Miller's Twist Rule
Caliber 0.224 Inches
Bullet Weight 60 Grains
Bullet Length 0.832 Inches
Barrel Twist 9 Inches/turn
muzzle velocity 2800 fps
Temperature 59 degrees Fahrenheit (59 is standard)
Pressure 29.92 inches of mercury (29.92 is standard)
Sg = 1.81
Sg shouldn't be less than 1.4. If Sg is greater than about 2.0, you may
gain some accuracy by going to a slower twist barrel.
Once a bullet is fully stabilised, precision depends on other factors and the Sg value obtained is of relatively minor importance. Whether a bullet performs in your barrel depends much more on its manufacturing quality, its inbuilt design factors such as bearing surface length, dimensions and form and how well they match your barrel's bore and groove dimensions and characteristics, the consistency of the powder charge burn and how well the vibrations that the bullet / charge produce suit the action's and barrel's harmonics.
If you're considering very long-range shooting, the retained velocity at the target allied to bullet design is crucial as moving into transonic velocities then through the sound barrier in to subsonic speeds can cause severe turbulence to some bullet shapes leading to yaw and instability - why the 168gn Sierra MK usually performs poorly in .308W 2,600 fps loadings at distances beyond 800yd for instance. This is not a twist rate issue, although they are not entirely separated. If you Google 'Berger OTM' you'll find a load of information on the newly released 0.308" 175gn Berger Open Tip Match 'Tactical' bullet designed for complete transonic long-range stability, but in a 1-11.25" twist rate barrel, ie to suit 2,600 fps loads in 20-26" barrel .308s.
I hope these observations help rather than hinder.
Laurie,
York, England
