The locking shoulder is the little pin with a flat spot on it and the kidney shaped head. It presses into the receiver and the back of the bolt is cammed down in front of it when it is locked up. The flat spot is what the bolt pushes against when it is fired, so the thickness of it at that point determines how tight or loose the headspace is. They come in various sizes and the proper size one is calculated and installed during the build. You might want to go over to the FALFiles website and see if GunPlumbers build manual is still available. All the FAL build info you will need in one place, IIRC he also has a video out as well.