The main reasons for why your lock nut will come loose are:
1. Not enough torque applied, especially with loose/undersized threads on the RET
2. If you use an end plate with sling loops or a QD sling swivel socket and mount a sling at that point, which pulls on the plate.
With the Lock nut installation jig, that clamps to the pistol grip boss on the lower and keeps the RET aligned TDC for the stock shaft, you can really crank down on them and get the torque value plus alignment.
I prefer using Blue Loc-tite, sufficient torque, and never attach slings to the end plate.
We’re already dealing with what is essentially a prototype quick solution from about 1965 for the first XM177s that hasn’t been replaced with something superior, because it actually works pretty well. As soon as you apply forces to it that it wasn’t meant to handle, it might come loose.
Q did an interesting video on this recently:
