Lug Lapping
Lapping a bolt's locking lugs so both bear evenly against the action, spreading the firing load and improving shot-to-shot consistency.
On a bolt-action rifle the locking lugs on the bolt rotate behind matching seats in the receiver to hold the chamber shut against firing pressure. From the factory, the two lugs rarely contact their seats equally, so one lug may carry most of the load while the other makes only light contact. Lug lapping uses a fine abrasive compound and a lapping tool to cut the high spots until both lugs bear evenly.
Even contact matters because it keeps the bolt from cocking slightly under pressure, which would let the case head shift and add inconsistency to each shot. Lapping is commonly done as part of blueprinting an action, and because removing material moves the bolt face rearward, the smith rechecks headspace afterward to keep it within safe limits.