Just reading this now, I built a .308 AR using a Krieger barrel, 26" long, exceptional accuracy bordering on 1/4 MOA. If long range is what your looking for, I've loaded 185 Berger Juggernauts to 2700 FPS for shooting 1000 yards. Shorter barrels won't make enough velocity to stay supersonic at that range. For reliable 1000 yard shooting you need to stay out of the transonic range, about 1300-1350 at the target.