.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 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.