Has anyone out there actually done this and if so could you write a short precedure?
First off headspace and locking seating depth are two different things.
To start off, check the chamber with a headspace gauge with the bolt on the back of the gauge**. There should be around .005-.010 gap between the two if the chamber headspace is correct.
Once you are sure that the headspace is correct, and then it’s time to set the barrel. With the front of the trunion/receiver pointing down, load the barrel muzzle in the trunion feed the barrel threw until you get to the barrel shoulder. Using a press start driving the barrel shoulder into the trunnion/receiver. Using the bolt/carrier, load the bolt into the trunnion and press the carrier down to load the bolt rollers into the indents in the trunnion. When the barrel is pressed into the trunnion the correct depth, there will be a gap between the back of the bolt and front of the carrier(bottom) of .018*. If the gap is greater, then the barrel needs to be pressed in more, if less, then press the barrel all the way out and start again. The father that the barrel is pressed in, the smaller the gap.
Once you have the barrel set, mill the slot for the receiver/trunnion cross pin and press the pin in. If the trunnion is all ready welded into the receiver, then all you need to do is use a laser down the bore and set the front sight, if not then you will need to jig the barrel trunnion up true and weld it in, then install the front sight.
* When you set the gap to .018, the first shot threw the barrel is going to drive the barrel forward against the cross pin. This will shrink the gap down to .014 -.010 depending on how tight you milled the slot. If you just set the gap to .012, then fire the rifle, the gap shrinks down below .008 and below the working spec.
**If you are using a new FAC barrel, you have your work cut out. The first thing you will need to do is take a finish reamer to the chamber to clean it up. Then you will need to set the breach back to get the correct head spacing, then take a file to the flutes to make sure that the flutes are continuous threw the neck section, and the flutes side edges are dulled or they will grab the brass and cause the case to stick in the chamber.