taken from Beartooth Bullets website and it works.
Note by Veral. Since all this was plagerized from my material, by Beartooth, Marshall Stanton, with minor changes, notions and errors, I'll leave it mostly in place but remove the errors and or explain the error and initial that portion. Understand that the discussion here is about slugging the rifled part of the barrel only. Slugging the throat is an entirely different process, for entirely different reasons, and every bit as important, whether for revolvers or rifles. LBT sells throat slugs for that purpose, and push through slugs for every caliber US and metric, for feeling out a barrel. (slugging in Marshalls words)
Slugging the barrel involves driving a soft lead sinker etc thru the barrel from muzzel to chamber and measuring the slug afterwards. When slugging you will feel the constrictions in the barrel left by machining, stamping and dovetail cutting. (Truth, VS. Sinkers or lead balls can work, but are very crude and change diameter every time they are tapped as they are driven through, giving a false diameter and a totally inaccurate 'feel' of the barrel. LBT push through slugs, which are designed for slugging barrels with utter precision, require one little bump on a cleaning rod with the hand, to get started. The slug is then pushed through by hand, with a cleaning rod, and reveals every little change inside the barrel, to one ten thousandths of an inch. By contrast, any slug that is driven through can change diameter by several thousandths with each tap, which swells it to whatever size or shape the barrel is at the point where it is tapped. They cannot be slid throgh by hand and give a feel of what the bullet feels.)
"Fire lapping very basically involves shooting a soft, BHN 11, oversize bullet impregnated with a high grade lapping compound (Note by VS. 'high grade lapping compound?? I developed what I believe to be the best compound possible for the process, and make it myself. This because commercial compounds don't have a high enough concentration to make the lapping process fast, nor the assured purity to eliminate undesirable scratches in the bore. The LBT lap kit supplies enough of this compound to do 25 to100 barrels, and PROPER instructions for doing it, WITH ABSOLUTE MEASURED PRECISION, which is the only reason for lapping..) 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. (Note, VS. The Beartooth Technical Guide is what caused the falling out between Marshall and me. Marshall asked if he could quote a few parts from my book Jacketed Performance with Cast Bullets, and I permitted it IF HE NAMED THE SOURCE. But he instead plagerized the entire work, which is based entirely on my own many years of experimention, research and knowledge. I wouldn't be so mad had he left it intact and told who's information he was giving out, but to change facts with inaccurate opinions and claim it's his I can't stand. If you do business with Marshall you'll find him corrupt quickly enough.)
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. (Note, VS. Fouling collection is of absolutely no concern. Fouling PRODUCTION is the problem which needs solving, but is a small part of the lapping process, which is in fact to make the barrel an ultra precision bullet launcher.)
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." (Note, VS. The statement "most barrels today are .0005-.0015 over nominal", is complete error. Manufactures tightened up tolerances dramatically soon after my lapping process was made public to thousands of gun owners, and complaints got heavy. Modern production, ie since approximately 1994, will tend to be well undersize from most makers. But EVERY BARREL IS AS INDIVIDUAL AS WE PEOPLE ARE! Any man who knows precision and has measured several thousand barrels will tell you this.)
I hope this helps,
Dave 
