I am going to build a LH 6.5x55 on a Ruger MK II action. Twist rates vary from 1-8" to 1-10". This rifle will be used mostly for big game hunting using bullets from 125g to 140g. I suppose I might shoot 100g or 120g BTs at coyotes with the rifle as well.

Can anyone speculate on the best twist rate for this application?

one in 8 or one in 7.....

your twist selection needs only be concerned with the longest bullet you intend to shoot.

If you're happy with the 140 grain as max then a 1-9" will suffice.

I would go with the faster twist rate. Many 6.5 x 55s also shoot the lighter weight bullets quite accurately in spite of the long throats and fast twist rate the 6.5 x 55s normally come with...Rusty.

SOME 9's stablize bullets over 129 gr, but I have seen those that did not, having owned them, and would only use a 7-8" twist, and Rusty is correct.

I have two 6.5mms, a CZ550 FS in 6.5x55 and a Kimber 84M in .260 Rem. The CZ has a 1/8.6" twist and thrives on Hornady 140gr SPs. The Kimber has a 1/9" twist and prefers 129gr Hornady SPs. If I were to build a custom 6.5mm in either of the two aforementioned chamberings, I would pick a 1/8" twist to handle long bullets in the 140-160gr range without any doubt.