I built a rifle up for a buddy a couple months back using all PSA parts. He has since told me that the selector will not go into the safe position when the bolt is locked back. I had a chance to put my hands on the gun last weekend and I performed the following trouble-shooting:
I removed the upper, cocked the hammer, and easily switched the selector to the safe position. I switched the selector back into the fire position and then, while applying downward pressure to the hammer, attempted to move the selector to safe and it would not go.
So, is the selector out of spec or is there some weird interference with the bolt carrier?
Ok, check the back part of the trigger (that interfaces into the selector). Make sure that it isn’t too fat or have a burr on it. Then look at the selector slot (where the trigger rides). My guess is that it is not fully machined or there is a burr in there.
Take another FCG and stick it in the gun or even easier, pull the selector and put in a known good one and see what you get.