I though I saw 14" twist on a box of the Berger 155 VLD hunting several years ago.
Someone please correct me if I'm wrong and if they changed that.
The bar on bullet stability requirements has been raised since that bullet first saw the light of day. Stability is measured using various formulae, a simple version available as 'Millers Twist Rule' donwloadable as a simple spreadsheet. Results are quantified as an single Sg (coefficient of stability) value. In theory, anything that works out as 1.00 or better is stabilised, in practice a result of 1.4 was seen for many years as optimum. However from recent research, this has been increased to 1.50 or higher for full stabilisation. The 155 Berger VLD measures out at 1.247" OAL according to Bryan Litz (
Performance of rifle bullets 3rd edition) although no doubt that will vary by a few thou' lot to lot. (Bullet length in relation to calibre is the key determinant of the required spin rate and hence rilfing pitch.)
Put that into Miller at 2,800 fps and you get
1:14 twist: ............ 1.09
1:13 ..................... 1.27
1:12 ..................... 1.49
1:11 ..................... 1.77
1.10 ..................... 2.14
So 1:12.4 twist pitch was required to meet the old (1.4) target figure, and 1:11.9 the revised (1.5).
These results are for 'standard' meteorology / ballistics conditions - ie 29.92 inches mercury air pressure and 59-deg F air temperature. Should you however only shoot at Raton NM in summer, you get much higher / better results. IIRC, Raton is ~7,500 ft ASL and air pressure drops by around an inch for every 1,000 feet altitude above sea level. So, if the base pressure of 29.92 applies at sea level on a day, it'll be around 7.5 inches lower up in the Whittington Centre, ~22.5 inches. Let's say temperature is running at 80-deg F. You now get Sg values of:
1:14 twist: ............ 1.51
1:13 ..................... 1.76
1:12 ..................... 2.06
1:11 ..................... 2.45
1.10 ..................... 2.97
So, the 155 VLD is fully stabilised in a 13 twist at 2,800 fps even for the revised requirement, and a lot depends on where you actually shoot under which conditions.
FWIW, I once tried a combination in 308 Win that gave a Miller result in standard conditions very close to that for the 155 VLD in 1:13 - 1.07 in my case. This shot very, very well in load tests at 100 yards, but in a match a few weeks later on the same range but under rather colder conditions at 800 yards, the bullets wouldn't hold the GB F-Class 3-ring (US 8-ring) and were producing up to 2-MOA variations.
