Lapping the bolt is done to improve the contact between the bolt lugs and the surfaces that they bear against on the receiver. Good contact will minimize the increase in headspace over time.
Shimming the gas cylinder can be done for a couple reasons. One is to make the gas cyl lock stop at the 5-7 o'clock position (a concern for match rifles). It can also be done to pinch the barrel band in place so that it doesn't rattle on the barrel (a concern for accurate shooting, but on match rifles, the band is screwed or welded onto the gas cylinder). Lastly, you can do it for optimal alignment of the barrel gas port and the gas port for the gas cylinder.
Neither one affects reliability.
Shimming the gas cylinder can be done for a couple reasons. One is to make the gas cyl lock stop at the 5-7 o'clock position (a concern for match rifles). It can also be done to pinch the barrel band in place so that it doesn't rattle on the barrel (a concern for accurate shooting, but on match rifles, the band is screwed or welded onto the gas cylinder). Lastly, you can do it for optimal alignment of the barrel gas port and the gas port for the gas cylinder.
Neither one affects reliability.
