I'm new to long range shooting (since Nam), and I'm trying to decide what type of upper barrel assembly to buy. I will scope the rifle, so open sights are optional, but I want to use the AR-15 as a personal protection weapon, as well. My options are a 24" or 20" barrel. Will the 24" barrel give me enough extra accuracy to overcome the lack of maneuverability of a shorter barrel? Also, I'm deciding on a stainless steel, bull, or normal chrome-lined barrel. What are the advantages of Stainless? Another option is twist rates, as I noticed a difference between 1/7", 1/9" or 1/12" twist rates. Does the length on the barrel determine the best twist rate? I have a budget, so I'd like to keep the cost of the upper assembly under $900. Thanx for your help, Dante'