Most premium barrels are lapped. The unanimous verdict of just
     about every expert in the barrel making arena is that lapping does indeed
     improve accuracy. Lapping accomplishes this by polishing the interior
     surface smoother and eliminates any tight spots in the bore.

 Lapping is generally a hand operation. A rod with a handle at
     one end is inserted into the new barrel. Molten lead is then poured down
     the muzzle around the rod for a distance of around 4". The lead then
     hardens and the lead plug or lap is tapped out. The lap now has the
     perfect relief pattern of the interior of the bore. Lapping oil or lapping
     compound is placed on the plug which is then passed back and forth in the
     polishing process. An experienced person can readily feel any tight spots
     and then work to eliminate them to produce a near dimensionally perfect
     bore. Again, all this hand work costs money, so only the very top quality
     barrels will be hand lapped.

    Now you might be thinking, "Well I can improve the accuracy of
     my unlimited gun by lapping the barrel my self". Don’t do it. Lapping will
     increase bore size. The barrel maker has already taken this into account
     when ordering his buttons so when he laps, the final diameter will meet
     specification.

    Lapping a mounted barrel will especially increase the bore
     diameter at both ends of the barrel. This isn’t a problem for a new barrel
     since one end is going to be crowned and the other chambered. However, on
     an existing mounted barrel which already has a chamber and crown, throat
     diameter will be enlarged and so will the muzzle. This is especially
     critical at the muzzle because it’s very possible that the bullet won’t be
     fully supported when it exits and will tip and yaw excessively as a
     result. Accuracy is likely to be very poor.

    Let me end this piece by making a comment about breaking in a
     new barrel. I’ve written about this in the past so I won’t repeat the
     details of the process and will just say that if you want the very best
     accuracy that your barrel can deliver, you should take the time to follow
     a break in procedure. Why? Because even the very best barrels will have
     small imperfections on top of the lands and in the grooves. When you
     follow a break in procedure, you’re giving your barrel a final polish that
     will smooth out these imperfections, and your new barrel will be shooting
     its best in the least amount of time. Accuracy reducing fouling will be
     also minimized. I know it’s a pain, but it’s a good investment in future
     accuracy.

    So there you have it. If you keep this information in mind when
     shopping for an aftermarket barrel, you’ll be more likely to make the best
     choice for your needs. A good barrel is like a good dog. They give so much
     pleasure that you’ll remember them long after they’re gone.