I have lapped a few barrels BUT only for Centre Fire rifles, 303's & 30-06's. All performed well after lapping. I used lead laps on a rod but in the last 20-30 years some people have used PRESSURE LAPPING, coating bullets with various coarseness abrasives and shooting them. (Tried this only once using JB, as mentioned above). If I remember, barrels that have been PRESSURE (or FIRE) Lapped have the throat lengthened so it's a tapered result. I'm not sure this would improve an air rifle or not. I've recently tried shooting different sized pellets, as mentioned by Grauhanen, and am happy with the improvement from selecting a "better" size.