We get inquiries almost daily about "truing an action" or "lapping the bolt lugs". The only time this should be done is when the barrel is to be replaced. This allows the job to be done properly and completely.
Taking a few thousandths of an inch off of the front of an action to make it truly 90 degrees to the bore reduces the headspacing of the barrel. To correct this the chamber must be reamed out the same amount as what was taken off. The problem here is that the reamer we use may not have the exact same dimensions as the one used to cut the original chamber so there is a good chance of ending up with a less than perfect chamber.
Lapping bolt lugs also removes some material from both the lugs and the action. This increases headspacing. To correct this the barrel has to be set back.
To do a full and proper truing of the action, the front face is machined down a few thousandths of an inch while the action is supported with a precise mandrel through the bore of the action. While in this mandrel the threads are checked for concentricty to the bore of the action. If they are out of true center the threads are then cut deeper until they are truly in the center of the bore. This makes re-using the factory barrel impossible as there is no way that is practical to increase the thread diameter. Many shops do not recut threads for this reason. We only true an action when we rebarrel it, this way we can cut threads that match up perfectly between the action and the barrel. 90% thread contact is our goal, however 65% to 75% is the industry standard.
The bolt lug face is then recut to a perfect 90 degrees to center of the bore and the bolt lugs are then lapped for full contact. Rather than surface grinding the factory recoil lug that sits between the action and the barrel (on a Rem 700 style action) we find it more cost efficient and beneficial to simply replace the lug with one of our precision machined lugs that we make in-house. These recoil lugs are nearly double the thickness of the factory supplied ones and are surface ground flat to less than a .0005 tolerance.