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Actually, the video that JB2012 shared is the exact one I looked at, forgot to add that I did the blue loctite deal also. I'll be doing it on every one I set up from now on.

I figured I would share a story about an AR build that was giving me fits, and how I fixed it. I recently got into building AR's and was on my 3rd one when I ran into a major accuracy issue. I had bought a 16" new M4 upper that already had the barrel installed. The upper would shoot about 3" groups at 50 yards with M855 ball with a properly mounted scope. I have never had one shoot this bad, in fact, of the 2 I had built previously , each shot slightly under 1" at 50 yards with he same ammo. I pulled the hand-guards and gas tube and checked the barrel torque. It appeared to be around 50 ft/lbs which was in spec. I went ahead and pulled the barrel from the receiver and still found nothing that looked odd. I found a You tube video about upper receiver lapping, which I had never heard of. I went ahead and ordered a $30.00 AR upper lapping tool from Midway figuring I had nothing to lose. I lapped the receiver Friday, re-installed the barrel and test fired. What an amazing difference! The exact same barrel/receiver assembly is now shooting 3/4 - 5 shot groups at 50 yards. Just thought I would share in case someone else runs into the same issue.