Fire lapping very basically involves shooting a soft, BHN 11, oversize bullet impregnated with a high grade lapping compound through the bore of your firearm at airgun velocities. These abrasive bullets act like self conforming lapping plugs which incrementally remove constrictions in the barrel, uniform it dimensionally and smooth out the tooling and chatter marks in the rifling. Complete fire lapping step by step instructions may be found in the fire lapping chapter in the Beartooth Bullets Technical Guide.
In virtually all firearms it will either greatly reduce or totally eliminate both barrel leading and jacket fouling by smoothing out the tooling marks left in the barrel by the equipment used in barrel manufacture and by easing the sharp corners and edges inside the bore which collect fouling.
In most guns, there will be at least a modest increase in accuracy potential, to often times dramatic improvement in accuracy from increasing the uniformity of the bore dimensionally, and eliminating the constrictions which are common under dovetails in barrels, and barrel bands.
Especially in revolvers where the threaded barrel shank is screwed into the frame of the revolver, dramatic improvements in performance may be achieved. This is particularly true where cast bullets are used in the revolver, because the constriction under the threaded barrel shank (which can be from .002"-. 004" depending on make and caliber), acts like an undersize, bullet sizing die, thus sizing the bullet down under barrel groove dimension, creating an improper bullet to barrel fit. Not only does this condition deteriorate accuracy; it also promotes barrel leading. Fire lapping, properly done eliminates this conditionan.
Finally, fire lapping often times increases the nominal velocity achieved with a given load. This is accomplished by a reduction in the friction coefficient of the barrel. Lapping out even a small portion of the tooling marks in a barrel will reduce the friction coefficient of the barrel, at least to some extent.
A Note Of Caution
You will notice that the first paragraph about fire lapping specified the use of soft BHN 11 oversize bullets. There is a reason for this. The soft bullet will conform and swage down to the tightest part of the barrel, and not "spring back" to near original diameter due to its softness. Thereby the most aggressive cutting action in the barrel is at the point of constriction, and the remainder of the barrel basically gets no lapping action once that soft lapping bullet goes through the constriction. A harder bullet will "spring back" to some degree, and basically lap all parts of the bore at the same rate, thus enlarging everything uniformly, and not addressing the tight spots specifically.
I also mentioned oversize bullets. This is to insure equal lapping action not only on the tops of the lands, but in the bottoms of the grooves at the point of maximum barrel diameter as well. If the bullet is less than maximum groove diameter, it will only wipe off the tops of your lands, thus reducing the actual depth of your rifling. There is a popular practice out there in the shooting industry today to use jacketed bullets for lapping. Please use caution, and consider what I just presented, and the following concepts. Most barrels today are .0005-.0015" over what nominal accepted norms are for bore dimensions. Consequently, when lapping with jacketed bullets the only part of the barrel that gets lapping action is the tops of the lands and the bottoms of the grooves never get touched, which is where most fouling and leading occur, not to mention reducing the depth of your rifling by only wiping out the tops of the lands. Fire lapping isn't complicated, but it is tedious, and must be approached systematically with common sense.