I always hear people say to buy a complete upper to avoid headspace issues and I would like to ask what those issues could possibly be given the fact that AR-15 barrels are self headspacing. There is no adjustment that you can make that will impact headspacing in an AR-15. The only issue that can be addressed by the amount of torque one puts on a barrel/upper receiver combination is whether or not you have a good seat or if you are stripping the threads. The correct amount of torque on the barrel nut is 50 ftpounds. The only thing you have to remember besides that is when aligning the barrel nut with the gas tube opening is to tighten not loosen. I have built ARs for over twenty years and I was tutored by a retired COLT assembler and I'm sure of this information. I hope this helps.