Just a little followup for the guys that offered advice:
As I stated before I bought the squaring tool from Pacific Tool and Gauge
. ( I am a little hardheaded) I also bought my upper receiver and decided to go with a Vltor MUR upper. These are supposed to be a little thicker walled and well made.
I don't have a machine shop so the only way I knew how to check for square is with the squaring tool itself and the thread idea from OldTurtle . I lubed it up and worked it into the upper all the way to flush with the barrel nut threads. I then took a very thin piece of thread and tried to work it in between the upper and the tool to see if there were any gaps.
It was nice and tight all the way around so I kept easing the tool out and checking with the piece of thread and applying a fair amount of force to the thread as I did so trying to force it into any gaps. The thread showed the same clearance around the complete circumference of the upper face at various depths. I'm calling it square......
It is probably a waste of time.....but I know now, and if I check my next upper receiver and it needs lapping, the tool is my tool box. $30 for the tool did not break the bank.
I used both hands and pushed the thread into the seam 360 degrees. Just holding it here for the pic.
