Re: 1 in 9 twist with >70 grain bullets
1:9 should be fine up to 69gr or so. Thereafter a 1:8 or 1:7 is good. Go 1:8 if you can as 1:7 can wear down faster. Also speak to the manufacturer of the bullets you shoot. Nosler, Barnes, Lapua and Sierra, for example, really do know their bullets well.
Note that it's not bullet weight that determines rifling, but bullet length. We just assume that a heavy bullet is longer. Longer bullets need faster rifling rates to stabilize the tips. Go too tight however and you reduce your ability to shoot lighter bullets as with a 1:7 twist 40 grain varminter tips will fragment almost as soon as they exit the barrel. A bullet passing out of a rifle at 3200 fps, with a 1:7 twist is doing close to 330,000 rpm. In a 1:14 barrel it will only do half that, hence you need to be aware of the bullet design limits. Additionally, tracers go against the general weight length assumption as they are usually long for their weight. A 145 grain Orange tip 308 tracer is about the same length as a 168 grain HPBT tip.
My 1:9 XCR shoots 77 grain ok, but my 1:7 AR gets tighter groups and is better out at 200-300 yards. At 50 yards you can't tell the difference and at 100 you need to shoot several groups back to back to see a small group size difference. At 200-300 yards the group difference is more apparent. So, beware of choosing bullets based on 50 yard performance. Depending on the caliber/load/barrel combination the bullet may not have stabilized at 50 yards. Most of my 308s cavitate until about 50 yards and my 338 is not stable for at least the first 100 yards of it's flight path. As an example of this, it shoots a 0.5" group at 100 yards, but does the same at 200 yards. At 100 yards the bullet is still cavitating and this affects ultimate grouping.
