Originally Posted by
tangolima
This is my take.
Say the upper is clamped down. The barrel nut is torqued on to the threads on the upper, pulling the flange of the barrel extension to bear against the shoulder of the receiver. For any given torque from the wrench, it is balanced out by two components. One is the reaction force from the receiver shoulder to the flange, the other is the thread-thread friction, and the nut-flange friction. Clearly the 1st component is the meat, so we lube and do things to minimize the frictions. The nut-flange friction is the one that loads the index pin and breaks things.
We want the 1st component to reach certain level, and hence the torque spec. We need that force to have maximum bearing surfaces between the flange and receiver shoulder. Those 2 surfaces are supposed to be flat, but there are always high spots. Aluminum is soft. Enough force crushes the high spots. The torque is increased by 10 lbf-ft each time to limit the loading to the index pin. Each time some high spots are flattened and the nut can advance a bit further. It is sort of creeping forward action if you will. One will find the amount the nut advances diminishes as higher torque is reached, because the whole shoulder is now bearing, instead of minority high spots.
You were doing that except you didn't limit the torque increment in the last step. The pin slot got overloaded. The receiver shoulder did get flattened in the process. That's why you got alignment at lower torque (30 lbf-ft is actually below spec of 35 lbf-ft).
It is indeed good idea to lube whenever screw thread is to be precisely torqued. I prefer applying lube to the nut for this particular purpose, on the threads. Threading the nut in pushes excess lube to the flange.
-TL
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