Re: Will a 1 in 8 twist?
I believe that you may have some chance of stabilising the 90 grain Sierra MK, (about 1.160 inches)but getting eneough rpms to shoot accurately may be a problem.
I believe that you have no chance of stabilising enough to get accuracy with 90 grain bullets from JLK (1.225 in) or Berger (about 1.250)with the 8 twist barrel.
A few that I know found that the 90 grain JLK at about 3,500 from 22/6mm AI, would just stabilise, but accuracy was terrible.
A custom bullet maker told me about 5 years ago, that his experiments showed that the Krieger 7.7 twist was the minimum when using the 90 grain JLK in his 22/243 Middlestead.
I chronographed bullet weights from 65 - 100 grains in my 22/250AI to see what velocities the case was capable of producing. I got about 3,100 - 3150 fps with the 90 grain Sierra MK in a Shilen 24.5 inch 1 in 10 twist barrel. (Obviously they keyholed into target)
Hope that the above is of some help. Brian.