this is written on lilja site about lapping:
Cut barrel makers sometimes will contour a barrel before it is rifled. Since their method does not introduce any significant amount of stress to the steel it is not necessary to stress relieve the steel through heat treating. Some may choose to lap the barrels and some may not. Since the cut rifling process removes about 75% of the original surface finish left from reaming the bore, lapping may not be as important as it is with a buttoned barrel. Since some readers may not be familiar with the term lapping, I'll describe it briefly. A lap is cast inside of the barrel and is attached to a rod similar to a cleaning rod. An abrasive is "charged" into the lap, and it is then pushed and pulled through the barrel. The object is to improve the surface finish and make the size uniform from end to end. In practice, lapping is performed by hand and is a time consuming process, requiring a good deal of experience and feel if the results are to be successful. Another advantage to lapping is that the surface finish will be parallel with the rifling. Only the best quality match grade barrels are hand lapped. No production grade barrels are.
For those who think that lapping a production barrel or a factory barrel could improve its performance, I'll offer a little advice: don't do it. Lapping will wear both ends of a barrel oversize. If the barrel has already been cut to length and crowned as well as chambered, this would most likely cause more harm than good. The barrel would become oversized in both the critical muzzle and throat areas. Accuracy would probably suffer.
