When installing a new rifle barrel the chamber is cut to optimum headspace. Headspace is the distance from the bolt face to a datum point on the shoulder of the cartridge. Bolt lugs that do not bear evenly in the receiver locking recesses may wear quickly and set back. opening up this critical dimension. Accuracy and smoothness of operation may also be degraded. Lapping lugs is a common practice when tuning bolt guns, so what about semi-auto battle rifles like the M1A and M1 Garand? Same concept.
Checking lug fit is best done with the barrel removed so you can clearly see the locking recesses in the receiver where the bolt lugs live. Strip the bolt. Degrease the receiver and the bolt lugs and paint the flat bearing surfaces in the receiver with a marker or Dykem layout fluid.
You will need a tool to apply backward pressure against the bolt face while you work the bolt to rub off the ink and expose the high spots on the contact surfaces. A tool can be fabricated from an old barrel or you can purchase one from Brownells. If you are replacing a barrel with a high round count, the bolt lugs may have seated themselves through simple wear. Most M14 style commercial receivers with low round counts will require lapping.
Original government issue Garands will likely be more consistent, but they should be checked since the bolt you have may not be original to that rifle. Keep in mind that lapping will increase headspace, so it is generally not recommended when reinstalling a used barrel.
M1A and Garand receivers are very hard. You will need a lapping compound containing silicon carbide rather than the more common aluminum oxide or garnet types. Put some compound on the rear of the bolt lugs and install the bolt in the receiver. Install your lapping tool and work the bolt back and forth stopping short of allowing it to unlock.
Remove the tool and the bolt, clean the lugs and check contact as we did above. You will see the shiny bearing surfaces gradually grow wider as you repeat this process. Stop once they are close to 100 percent. We don't want to lap any surface except the flats. Checking lug fit and lapping if necessary should always be done when replacing a barrel or bolt.
Brownell's bolt lapping tool
The Brownell's bolt lapping tool. Norcross has used this one on dozens of rifles, and says it's the right tool for the job to get even lug bearing on both sides.
Lapping tool installed
Here's the lapping tool installed. You just insert a punch in the extractor hole to operate the bolt. Rotate it up and down, stopping short of letting it unlock.
The unlapped lug
The unlapped lug at left shows poor contact with the receiver, while the lapped lug at right shows close to 100% contact. Even contact is what you want.