The primary factors that determine the rotational speed needed to fully stabilise a bullet are:
(1) its length (as measured in calibres as a 1.25 inch long 0.224 cal bullet needs a far higher rotational speed than the same length 0.308, the former being 5.58 calibres long, and the thirty 4.06 calibres.) Small changes in OAL length result in disproportionate changes in twist rate requirements
(2) its weight. Contrary to common belief, if you have two bullets of the same form and overall length but of different weights, the heavier one needs less stabilisation, hence slower rotation and a slower rifling twist rate. Because a heavier bullet is also usually a longer bullet, weight was used as a shorthand method of advising twist rates. However as modern match designs have become longer and longer for their weight, this is now a poor yardstick.
(3) Form. A flat-base bullet needs less stabilisation than an equivalent boat-tail design. A good example is the Berger 88gn HBC Flat Base varmint and one-time match bullet. Berger Bullets advises a 10-inch twist for this model despite it having relatively long VLD type secant ogive nose section. The 90gn BT Match is recommended a 9-inch twist rate. The Miller Twist Rules formula is for BT designs and if used on FB bullets overstates the required twist rate
Sierra says the new 200gn MK has a nominal OAL of 1.585 inches. That compares to the 'old' 7-calibre radius ogive 200gn SMK's length of 1.387". That's less than two tenths of an inch increase, (0.198") or 14.3 in percentage terms but as noted, a relatively small length increase in the same weight / calibre bullet has a disproportionate effect on stability requirements.
Assuming a 308 can give it 2,650 fps MV in a match rifle and running that through the Miller Rules spreadsheet or equivalent you get Sg values for different twist rates of:
10 ............. 1.35
9.5 ............ 1.50
9.0 ............ 1.67
That's under standard ballistic conditions (ambient temperature 59-deg F; atmospheric pressure 29.92 inches of mercury which is the meteorological 'standard' for sea level on an average day). Shooting at high altitudes (reducing atmospheric pressure) and/or in high temperatures will increase those Sg values giving you more latitude on twist rates. Shooting at sea level on a day below freezing with a high-pressure anti-cyclonic weather system raising atmospheric pressure has the opposite effect and reduces the values so higher rotational speeds are needed.
Assuming a 300 Win Mag can push this bullet at 3,050 fps, the three twist rates see:
10 ............. 1.42
9.5 ............ 1.57
9.0 ............ 1.75
(To show how bullet length affects Sg, all other things being equal, here are the values for the old 1.387" OAL 200gn SMK at the 308 2,650 fps MV under standard conditions:
10 ............. 2.00
9.5 ............ 2.21
9.0 ............ 2.46
An 11-twist barrel is fine for this bullet giving an Sg of 1.65)
I don't want to tell people what they already know, but I suspect Fatboy is asking what is this Sg thing and how do you calibrate it?
Sg is the coefficient of stability. In theory, a bullet is stable (ie will not tumble in flight) with an Sg value of 1.000. In practice this has been found to be much too low for both accuracy and to account for the wide range of conditions people shoot in. (The prototype Stoner XM16 rifle used the old 222 Rem's 14-inch twist until the US Army shot it in its Arctic Warfare School ranges at Thule in Iceland in winter where it couldn't hit the required 500 metre target at all never mind group - the XM16E1 follow-up version changed to the 12 inch twist for that reason.)
For many years 1.4 was the recommended minimum Sg. In practice, Sg values of 1.2 and above would see bullets group OK and apparently perform well especially at short range. However about five, six years ago new research showed that a minimum Sg of 1.5 was needed not only to stabilise the bullet at all ranges but to obtain its listed BC value. Or to put it the other way round, twist rates / rotational speeds that were below the 1.5 threshold increased drag slowing the bullet quicker.
So, going back to the new 200gn pointed MK at 308 Win velocities, the 10-twist Sg (1.35) will see the bullet group OK at most likely all distances in summer, but it will increase its drag / reduce its BC from that claimed by Sierra. (The Berger Bullets' Twist rate calculator
http://www.bergerbullets.com/twist-rate-calculator/
not only calculates Sg values but if they fall short gives an estimate of the BC reduction. Taking the similar but shorter Berger 200.20X bullet at 2,650 fps, a barrel twist rate of 10.7 inches produces the 1.35 Sg value for this model and the program says that reduces the BC by an estimated 4% from 0.328 to 0.314 G7.)
The 9.5 inch twist rate is right on 1.50 Sg so should suffice, but no doubt Sierra has reduced the requirement (or increased it in reality) to a 9-twist to give greater margins for adverse conditions.
There is another factor in these new generation super-long bullets - the issue that intrigues Fatboy, ie what is different about these high-BC bullets. It is as said in other posts that the nose sections have been made longer and longer and other bits, primarily the central bearing surface section have been shortened to compensate. This reduces the dynamic stability of the bullet (different from gyroscopic stability which rotating the bullet in the rifling achieves) making it likely to be overall less stable in flight especially at long ranges where velocities drop into the turbulent transonic zone (MACH 1.2 to 1.0). Some extra gyroscopic stability is no bad thing for the FTR shooter who competes at 1,000 yards.
Bullet nose shapes are described in terms of a radius in calibres. The original 200gn SMK is measured at a radius of 7.30 calibres (part of a circle whose radius is 0.308 X 7.3 = 2.25"). if the new high-BC model is as per the rest of the new generation MK family, that has increased to a whopping 28-calibres or thereabouts. 28 x 0.308 = 8.6" radius circle giving a very much longer and much less sharply curved nose. That reduces drag but creates a much more finicky design to tune in the barrel and makes life difficult for the designer to make the bullet 'well balanced' in its flight performance at all speeds in all conditions. Read Bryan Litz's Modern Advancements in Long range Shooting Vol II and you see that Bryan is starting to think in terms of yet tighter twist rates and higher Sg values for the ELR or other shooter where transonic speeds are issues. He does tests with 308 with an 8-twist rifling rate barrel and sees improvements.
Personally, I'm beginning to think that we're in danger of looking through the wrong end of the telescope and that instead of moving to six-inch twists it may be that we need to stop making bullets ever longer especially for use in 308 Win. The 200gn new SMK is longer than the Berger 200.20X - I'm waiting an watching now to see how well it performs throughout a season or two. BC isn't everything!
