That makes sense. I am sorry if I miss-interpreted the post.
According to the old greenhill formulas, the 168 became standardized for the 12 twist 308 barrels. Also this is where the 11.25 was born for the 175. Greenhill has always been on the fast side of things and only takes into concideration bullet diameter, length and twist and only two velocity ranges. The problem is that going from 2799 FPS to 2800 FPS isnt a magical jump. More modern and IMO more accurate formulas take into concideration actual velocity, weight, diameter, length and many other factors. These formulas along with real world proof show that the twists used by many of todays shooters work very well such as 12x for 175-180's and even 13 for 168's and 14x for 155's. On the surface, they seem too slow but in reality are very optimum twists for these bullets. Actually Berger bullets recomends 12x for their 190 VLD's and 13 for the 175's. This should help put into perspective how overkill many shooters are in their twist selection.
This is only my opinion, so please take with a grain of salt, but in my 308 experience, the 12x is a perfect ballance for the 175-180 where the 11.25 is perfect for the 190 SMK and the 11 is perfect for the 190 VLD's. The 10x is perfect for the 208-210 variety. Dont get me wrong though, these just seem like the optimum balances. You can still shoot 168's very well out of an 11x barrel even though 13 is more optimum. I have shot the 155's well from the 11.25 even though I prefer a 14x. It seems like when you use the optimum twist, you put the bullets to sleep closer to the muzzle than when you spin them too fast or too slow. It was taking 600 yards to put the 155 AMAX to sleep with the 11.25x where the 190's were put to sleep at the end of the barrel. This is my opinion simply due to the 190's shooting the same small MOA at 100 and 1000 and every range in-between where the groups with the 155's are horrible until after 600 yards, then they get better. This is why I am of the mind set "dont spin the bullet any faster than you have to". I say that 1/2 hearted because a bullet will be stable with a SF of 1.1 and 1.5 is the widely accepted optimum balance. If you reach 1.5 in normal conditions, then when you go to death valley (below sea level) during the next ice age (35 degrees below zero F) then your bullets will still be well above 1.1 and perfectly stable. This why we shoot for 1.5 in standard air. There is absolutly no need to try and reach higher than 1.5 unless you want more spin drift, more yaw and to see a bullets jacket imperfections manifest themselves on paper.
I dont meen to rant, I guess the point was to show where 12x and the 168/11.25 and the 175 standard came from. Unless you want to shoot bullets heavier than the 190's, I would highly recomend the 12 or 11.25X. 10x will work, but you will get better velocity with less pressure and stability factors closer to 1.5 with the 11.25 or 12x. If you need 208's - 220's then go with the 10x as that is what you will need.
