Ok; so after all this, I loaded up some 9MM last night, very conscious of making sure the primer was fully seated.
Part way through the run, one primer didn't feel quite right. Stopped and pulled the brass out of Station 2 (priming/charging on a Dillon 650). The OLD primer was still in the brass case...even though it had gone through the sizer die fully.
Toss the brass back in the hopper, and it happens again; old primer in the case and the new primer sitting all sad and lonely on the priming stud. I pulled this piece out and marked it with a felt marker, and dropped it into the feed tube. Sure enough, that piece of brass eventually shows up in Station one, and the priming stroke is almost zero force required. Pull that same piece of brass out of Station 2, and sure enough, the felt marker ink was there, and so was the old primer. This little surprise happened about 5 times over 175 rounds loaded.
I suspected the decapping pin was pushing the old primer +almost+ out of the primer pocket, and when that piece of brass rotated into the priming station, the old primer was pushed back into place by the new primer sitting on the priming stud. I Measured the decapping pin protrusion at 0.278". Measured the other decapping pins on other caliber tool heads and they vary from 0.300" to 0.340".
Oddly enough, this was only with Winchester brass. Even more odd, the force required to push the old primer back into the pocket was very similar to the force required to seat a new primer. Huh...
In the pics you can see the fired primer sitting fully re-inserted in the primer pocket, with the exception of one that I caught in station 1 before the priming stud and new primer pushed it back into place.
And here I was blaming CCI...