Well, I finished the lower, just need to torque the castle nut down and it's done. I have a prebuilt upper from Palmetto Armory and the Sharps Balanced Bolt Carrier Group with the Relia-Bolt and the NP3 PLUS finish. I've looked over a number of youtube vids about checking the headspace. I don't have the tools, and frankly, I don't totally understand the process. How critical is this?