For a 6mm, I would think 8" should be fine for up to 105gr bullets. If you intend to shoot 115gr bullets, I'd probably move to a 7.5" twist just to be safe.
Us high elevation guys can sometimes get away with 1:8" twists with 115gr bullets, but if most of your shooting is done at relatively low elevations, I would most definitely go with a 1:7.5" twist.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: M.Gordon</div><div class="ubbcode-body">8.5-8 twist is what we do all our 6.5 short action type calibers in. We use to do mostly 8.5 but the trend has been more along the 8" twist lines.