Have been trying to read up on powders for 308 as well. Because twist rate was mentioned here figured I'd copy and post this it mentions twist rate and projectile weight class observations in particular in the last paragraph of what I'm quoting. I'll also link the page itself if anyones interested in readin too to bottom what it has to say about various metrics: coal, powders, projectiles effects on the cartridge as well as a lot of other info.
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Barrel Length & Twist Rate
You'll see a wide range of barrel lengths and twist rates on precision .308 Win rifles. At one end of the spectrum, a tactical shooter who needs a compact, quick-handling rifle, may select a 20″ barrel. The popular Rem 700 LTR (Light Tactical Rifle) features a 20″ fluted barrel. On the other hand, Palma shooters, who need to keep a 155-grain bullet supersonic at 1000 yards, normally opt for barrels 30-32″ in length. The extra length also provide a longer sight radius–a big benefit when using iron sights. A shorter barrel definitely delivers less velocity, all other factors being equal. With most powder/bullet combinations, you can figure an additional 25-30 fps per inch going from 20 to 25 inches. From 25 to 30 inches, add an extra 17-20 fps. From 30-33 inches you may pick up 15-17 fps or so, with appropriate powders. Here are some velocity estimates projected by QuickLOAD for 44.5gr Varget (ADI 2208) and IMR 4064 with a 168gr SMK set at 2.800″ cartridge OAL, bullet not touching the lands.
QuickLOAD Projected Velocities in FPS with Increasing Barrel Length | | | | | | | | | | | | | |
Powder | 20″ | 21″ | 22″ | 23″ | 24″ | 25″ | 26″ | 27″ | 28″ | 29″ | 30″ | 31″ | 32″ |
44.5gr Varget (ADI 2208) | 2592 | 2620 | 2646 | 2671 | 2694 | 2716 | 2736 | 2756 | 2775 | 2792 | 2809 | 2825 | 2841 |
44.5gr IMR 4064 | 2628 | 2656 | 2681 | 2705 | 2728 | 2749 | 2770 | 2789 | 2807 | 2824 | 2841 | 2857 | 2872 |
Note: This is a software prediction only. Different barrels will perform differently based on a variety of factors including groove diameter, land configuration. Primers, seating depths, and powder lot variations can affect the numbers significantly as well.
A 1:10″ twist rate will let you shoot the popular medium and heavy 30-caliber bullets up to the 210-grainers from Berger and Sierra (provided your barrel delivers enough velocity for the heavy bullets). A ten-twist can also work well with the lighter projectiles. John Whidden, who won the recent U.S. Palma Team Trials, has had great success shooting 155gr Bergers with a Broughton 10-twist 5C barrel. A 1:11″ twist, found on many factory rifles, is a good compromise. Palma competitors using the Sierra or Lapua 155s have favored 1:12″ or 1:13″ twist rates, with the theory being that a slower twist rate may offer better velocity since there is less friction in the barrel. A 1:13″ twist is fine for deer hunters who prefer a 150gr-class bullet moving 2850-2900 fps."