For most 6.5's to shoot even 140gr. bullets a 9" twist is required. If you are looking at 140gr. TSX - the twist might have to be faster yet.

My Daughter's .260 Rem shoots everything from 85gr. to 160gr. into 1/2" or less, using a barrel made by Jeff Lawrence. It's truly amazing & has a 9" twist. The 120gr. XLC's it likes, are the same length as the 140gr. Interlocks it also likes. I do not know if it will shoot the 140gr. TSX or not. The 120's will handle everything she's ever likely to shoot, from deer to elk.

My 6.5x68 shooting Buddy put an early 120gr. "X" beside the bung hole on a mature (dry) cow moose at a range of just over 300yards. The bullet stopped just under the hide at the front of her chest, having lost 2 petals in it's journey. She braced her feet at impact and just stood there for 10 or 15 seconds, then collapsed. The 120gr. was just stable in his rifle with a 10" twist. The lung cavity was full of congealed blood when we opened her up. The lungs had a 2" diameter hole through the big part, with the bullet hitting exactly in the middle, disrupting both.

With the right type of bullet in a small bore, a light bullet can be just as effective as the heavier ones, or even more so. It's certainly a LOT flatter shooting.

My daughter's .260 makes over 3,000fps (26") with the 120's and having a couple gr. more capacity, the 6.5x57 should do the same, I'm sure.