Assuming either .223 or .308, 18inch 1/7 works very well for .223 and helps you keep the weapon a little smaller and lighter and should be able to engage out to 800yds without getting overly fancy, as for .308 20 inch barrel is more than enough just be sure you get a 1/11.25 or even a 1/10 and that would get you past 1000yds. I prefer the heavier bullets though 77gr in .223 and 168gr or 175gr in .308, thus I am a fan of the faster twist.