I just did a bit of research on 7mm twist. What I found comes from Dan Lilja, barrel maker extrordinaire.
: According to Dan, stability factors are given to bullets. A stability factor of 1.0 is required for the bullet to be stable at the muzzle. A higher number is advisable for hunting bullets which are not gyroscopically perfect. ie: their centre of mass doens't coincide with their centre of axis. Hornady's handloading books picture this quite well. Bench-rest bullets are more closely perfect in this regard & therefore BR shooters usually use twist rates that 'just' stabilize the bullet as this stability shows best potential accuracy.
: In hunting rifles shooting hunting bullets, a slightly faster rate of twist will help to overcome minor deviation in the bullet's 'less than Bench Rest perfection'. A stability factor of 1.5 has been suggested. Too, we are looking for stabiity under hunting conditions that may involve shooting through minor structure as in bushes or branches. To maintain stability through these obstacles and during penetration of animals as the bullet expands, faster twists are required than what gives the very best accuracy.
: The bullet used in the testing was 160gr. flat base, with an overall length of 1.270" - a fairly long bullet.
: Testing showed that at 2,700fps, a normal 160gr. velocity for a 7mm 160gr. bullet, a stability factor of 1.5 was met with an 11.1" rate of twist. At 2,700fps, the stability factor for the same bullet from a 12" twist was still 1.32.
: Thus, we may use any twist faster than 12" and expect good accuracy. The faster the twist, the less 'inherant' accuracy potential, but the better stability for hunting purposes. These tests show, for the long pointy 160gr. bullet the twist rate doens't need to be faster than about 9", but we do know that the more stable the bullet is, the better it maintains it's stability after impact.
: This also shows that there is no bullet commonly used in 7mm calibre rifles that would require faster than a 9" twist, so 8" or 8.5" might not be of any benefit.
: With a static imbalance of .000250", the difference in potential group for this same 160gr. bullet is: 12" twist = .453" group and for a 9" twist, .604" group. This accuracy 'loss' is easily absorbed in hunting conditions and become meaningless.
: This may sound too complicated, but is merely meant to show that about any twist used today in a 7mm barrel is sufficient for hunting bullets of any weight as most twists are 10" or faster. A 10" twist will stabilize that 1.270" long bullet down to less than 500fps and up to in excess of 5,000fps. Need more? : If anyone has more experience than Dan Lilja, perhaps they can add or subract from these figures.
: The 175gr. Hornady Interlock measures 1.358" long, some .088" (88 thou) longer than the 160gr. used in Dan's testing, but is also well stabilized by any twist faster than 10".