Lapping is the use of an abrasive and actually removing metal out from the inside of the bore. Some people including myself have used valve grinding compound. Others have used Comet Cleanser. J.B. Bore Paste is an actual mild form of lapping although it is not as drastic as other methods. Another method is using Scotch Brite Scrubber pads and working them back and fourth in the barrel....

Then there is the fire lapping method. Where you take conicals and actually lube them with a valve grinding compound, push them down on a charge and fire them. The results is the compound removes a thin layer of metal from the barrel. There are also professional fire lap kits.

Lapping is a way to smooth out all the ruff spots inside the bore. The problem with lapping is before you make this commitment to your rifle remember... once you take it out, you can not put it back. There is no guarantee that lapping will solve your problem. It is a last resort. Although it does wonders for a stubborn rifle....