When I build a rifle, truing and timing the action is a standard procedure. Most shooters understand the concept of truing, but are confused about what "timing" is. When an action is out of time, it is typical that the bolt operation is stiff, bolting or unbolting. This is caused by several things, such as the cocking ratio being out of balance, and the sear engaging too early or too late. Another geometry problem is that the lead-in ramps and the primary extraction are not synchronized, but are actually in conflict with one another.
When we true an action, it consists of much more than just machining a few mating surfaces. First the lead-in angles (the ramps that lead into the lug seats) are re-cut to a true helical cam. The factory cuts are made with an end-mill, and are not helical. A helical cam is like a spiral staircase, and I have developed special tooling to precisely cut the lead-in angles to engage the whole width of the bolt lugs. The receiver face and the lug seats are machined to square them up relative to the receiver threads, not to the bore of the receiver like most gunsmiths would do using conventional wisdom. The reason for this is the barrel will be square to the bolt face. Concentricity behind the bolt head is not critical, as the "float" in the bolt head will allow the bolt body to be off center as much as .045" in any direction, and still get full lug contact, although I've never seen one that bad.
The bolt head is squared, front and back and the firing pin protrusion is reset to .040" +/- .005". After this is done, the firing pin travel is corrected and timed with the sear and trigger. At this time the cocking ratio is corrected to avoid over cocking and hard bolt lift. The cocking ramps are reground to the correct angle, and lastly the barrel nut is faced for 100% contact.