Unlike many others, lapping a barrel is something I will often do. What's more, I have no trouble with lapping a barrel after chambering. For some reason, barrel lapping has come to be regarded as a mysterious operation which is more likely to cause harm than do good.
There is little doubt that many people should not even consider attempting to lap a barrel. Let's face it, many people should never be allowed to handle anything harder or sharper than a Twinkie! It is possible to screw a barrel up with a lap just as it is possible to wreck a paint job with a buffer. It doesn't mean this is the most likely result however.
Mostly, lapping is a means of refining the interior surface finish of the barrel. It can also be a means of improving dimensional uniformity or correcting dimensional deficiencies but, generally speaking, these don't usually exist in good quality barrels. If a maker of BR barrels produces a barrel with a noticable tight spot, he will probably not try to correct this with a lap but will cull the barrel.
In the days when muzzleloaders ruled, lapping was often employed to produce a choked bore as a means of easing loading and, supposedly, improving accuracy. I don't think there are many advocates of the choked bore today so this has fallen out of favor.
I have found it is entirely possible to do all sorts of things which most say not to do. For instance:
Don't remove the lap from the barrel. You'll never get it back in place. I do this all the time. In fact, I will index the lap around in an attempt to uniform the land and groove dimensions.
Don't lap from the muzzle end. I actually find this to perhaps be the best way to lap the throat area.
You can't lap gain twist barrels. Because I don't appear to know any better, I lap them all the time. Works good.
I consider firelapping to be a technique for the inept.
Now, as to bored184's question. He states, "I've got a Kimber 82G that shoots great". This hardly sounds like a candidate for any type of remedial work whether it's lapping or rebedding or anything else. If a rifle "shoots great" I'm usually pretty happy with it. Lapping is something that is done to barrel which has an identifiable deficiency or as part of the manufacturing or the chambering process. It isn't done as part of a barrage of changes and alterations or as an act of desperation. As gunsmiths, we should have some idea of what it is we are trying to do before we do it. Regards, Bill.