Had this happen once with my NM. I was putting all kinds of dope on it. Went downhill from there shooting the Texas State Service Rifle Match. In desperation I decided to CLEAN it. Guess what - the front sight was loose.

FWIW I have an immaculate HRA with a new-appearing barrel. Arrington did a JCG accuracy job on it. Groups walk all over the place. My only M-1 that does that.