Thanks, Bill. Using your numbers and definitions with Coinneach' formula, the twist would be 9.9 compared to your 8.25--due to the 180 vs. 150 as the Constant.
So the formula works; the argument now is about what Constant to use...Off the cuff, since the 117-grain bullet in .257 is rather long, it would appear that using 150 as a constant might work better. I'm guessing that a 1-in-8 twist would be more stable than a 1-in-10 for that bullet.
Ain't it fun?
, Art