If you are looking for distance accuracy I would go with the 20" 1 in 7 twist. Pair up with 75-77 grain BTHP's that will get you the most distance before you get key holing. Should make 500 yards pretty easy, unless of course, you want to go 22"- Law of diminishing return after that. Find some info on the Marine Marksmanship team, they did some innovative work with the 5,56.