Twist rate has more to do with .308 predictable hits at distance than barrel length, in addition to BC.
You want tight twist, and high BC, with a consistent load.
Todd Hodnett prefers a 16" bull barrel with at least a 1/9 or tighter twist with the .308 gas guns.
Even his son was out-shooting 24" .308 bolt guns at a mile with the little 16" carbine with custom barrel, shooting 175gr SMK's IIRC.
When I was chasing this demon before I wised up and ditched .30 cals, I was pissing in the wind at 800yds even in practical competitions, because the wind just takes your low BC .30 cal pill, and washes it downstream wherever it wants, while you continue to miss target after target.
Within 700yds, a well-built .308 with the right barrel and load can do very nicely when the wind is low. Once you get varying wind, it's time to know your limits.
Either way, go with a RLGS and a tight twist if you plan to shoot at those distances, and use either Black Hills or a great hand load. Prepare to be frustrated.
I was using 155gr Scenars, which have a pretty good BC, but fly much faster than the 168s and 175s, so you are flatter, with the same or better wind drift. I was using Lapua Doppler radar data, not theoretical drag models, so POI was dead on for elevation right out of the gate.
i have since gone to .260 Remington and 6.5 Grendel for these purposes. My 16" 6.5 Grendel has 1/7.5 twist, so it doesn't care about the transonic region, not that it gets near it within 1000yds anyway.