J&G, to answer your question about fast/slow rifling for heavy/light bullets: if the twist is faster, it will generally perform better with heavier bullets and slower will perform better with lighter bullets.
For the 6.5mm question, either the 1:9 or 1:10 would work well with 140gr bullets, and the 1:8 would probably work better for 160's.
For the .30-cal with a 1:10 twist, anything from 175-200gr should do well in this rifle. 200's may be pushing it for stability (they may be better suited by a 1:9.5 or 1:9 twist), and anything under 175 may "overstabilize" due to the twist rate, but that range should work well for you.
For the general rule of thumb, knowing (for each caliber) which bullets are considered "heavy" and which are "light," as well as the "middle ground" for each caliber really helps.
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