In setting headspace on a barrel.
Measure from the face of the action to the bolt face.
Measure the shank.
Subtract the shank length from the measurement you get from the measuring to the bolt face.
That is how much of the brass or headspace gage that should stick out.
You're suppose to make this about .004 longer to make up for the torqueing the barrel onto the action. I don't do that, I make it to fit as above which wont allow the "go" gage to work. I then hand turn the reamer one turn at a time until the bolt closes on the "go" gage.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071