I copied this from another forum because its right on the money and too lengthy a topic for me to hunt and peck my way through!
"True barrel lapping involves the use of a lead lap that is cast on the end of a cleaning rod by pouring molten lead in the bore. The lap is partially driven out of the bore (no more than half way) and an abrasive suspended in oil or grease is added to the lead lap. Be careful what you use for lapping compound. The desirable lapping compound is a very fine screen while Clover valve grinding compound is much more coarse and not suitable. The lap is then pulled the pushed, back and forth through the bore. Care must be taken as to now allow the lap to come out of the bore while pulling or pushing, or a new lap has to be cast. The larger abrasive particles are embedded more deeply in the lead than the small particles so there is an even bearing pressure from all the particles of grit onto the steel of the barrel. The net effect is to polish the barrel rather than scratch it, which would leave a matte finish.
Lapping the barrel adds between one and three tenths of a thousandth (.0001” - .0003”) to the bore and groove diameters of the barrel and is used by most custom barrel makers as the finishing process on the inside of the barrel. Lapping a barrel will improve the performance of almost any barrel - in some cases, startlingly so!
When the bore feels the same from end to end, the lap is removed and the bore is cleaned out with solvent to remove all the grit and oil from the first lapping job. A new lap is cast and a finer grade of abrasive is added and the job is repeated. This is repeated until the final grade of abrasive is completed.
Lapping will sand out the tooling marks left from creating the bore or the rifling. Just like sanding a piece of wood or metal, finer grades of abrasive are used to make the optimum finish. The process can be taken too far and polished too much. The barrel’s polishing lines should be parallel to the bore. This is thought to reduce the fouling to a minimum. A barrel with smooth lands and grooves will foul less than one with rough lands and grooves. The interior can be too smooth though. Makers of custom hand-lapped barrels spend a lot of time achieving the internal finish that they require. Though it may surprise some, lapping to a finer finish will result in an increase in fouling. A barrel can be too smooth.
This process can only be done to a barrel that has not been installed on a receiver. You should never lap an installed barrel except as a last resort! Reason being that a lap cuts more at the point where direction is change resulting in washing away the throat and muzzle. The abrasive particles are sharpest when they are first being used. When the lap is pushed and/or pulled the abrasive particles will cut more at the beginning of the movement (push or pull) than they will at the end of the movement. This results in a tapered bore. On new barrels that are not chambered, this is not a big deal as the cartridges chamber will cut out the slightly larger section of the bore. The gunsmith will then cut the barrel to length thus removing the other belled bore end.
Another process of lapping that is designed for installed barrels is called fire lapping. This involves firing abrasive coated bullets down the bore in increasingly finer grits. A company called NECO sells kits to do this. If you enjoy tinkering and reloading, you can get a kit for about $80 with instructions, bullets, and abrasives. Or you can spend $130 to get pre-loaded ammo. In addition to polishing the bore, it can remove minor manufacturing errors in the rifling, and has been known to produce remarkable improvements in accuracy. Fire lapping produces a tapered bore. The bore will be larger at the chamber end than it will be at the muzzle end. This is not always bad. Often choked barrels will shoot very good groups. The drawback to fire lapping is it moves the throat forwards. It will actually erode the steel moving the lands and grooves farther away from the bullet. How much erosion is dependant on how carried away the shooter gets during the lapping process."
