building my first ar. ive started my lower with spikes reciever, and components kit. im going to add a fully adjustable colapseable stock. looking for my upper components. im looking for 6.8 barrell. so much to choose from im a little overwhelmed. im most likely going with a 18 or 20 inch chromoly or stainless. twist rates from 1x9 to 1x11.bullet weights 115 or so. most my shooting will be at range 200 yrds, i want to build a real tack driver. any advise and suggestions welcome.