Lapping vs. Polishing
This subject probably needs a bit of detailing to get everyone on the same page.
Where I distinguish lapping from polishing is the former affects barrel geometry while the latter abrades and works the whole surface down. Firelapping, like hand lapping, is to remove identified constrictions or irregularities in the bore as well as tool marks to improve cleaning qualities. Such constrictions affect fouling in both cast and jacketed bullets. They generally affect the accuracy of cast bullets greatly and of jacketed bullets only a little. Polishing is strictly for smoothing the bore surface to reduce fouling or to clear up the alligator skin heat stress cracking in a throat. It does not remove constrictions significantly.
Whether your barrel needs lapping or not is determined by slugging the bore. Obviously a hand-lapped custom barrel has already been done. Other than lightly polishing the toolmarks in the throat, further lapping will be of no benefit. Determining whether a commercial grade barrel needs it or not requires slugging it with a pure lead bullet, ball, or fishing sinker. The bore is cleaned and a lightly oiled patch is pushed through. The lead object of choice is tapped into the rifling at the muzzle or at the breech with a dowel rod and mallet, then slowly pushed through the bore by hand, using a brass rod or a wood dowel. Pure lead is nearly perfectly plastic, so when it passes through a constriction it will keep the OD of the constriction and be loose in other parts of the barrel. You can feel the difference in how hard you are having to push it. Hard cast bullet alloy is not perfectly plastic and will spring back out after passing through a constriction, revealing no change in the feel of how hard the rod is to push. Indeed, I have run hard cast bullets through a bore then aligned the rifling back up in its original orientation and pushed it through again and found it just as uniformly resistant on both passes. .22 rim fire lead bullets are about as hard as you can get and still feel the bore properly. In my experience it is much easier to feel a constriction entering it from a looser spot in the bore than it is to feel a loose spot after exiting a constriction. For this reason I always slug a bore from both ends before I believe for certain that I have identified any tight spots.
Tight spots commonly occur on commercial grade barrels owing to there having been little or no stress reliving done to the barrel blank before it is contoured. When contouring is done, the places where the barrel is thinnest offer the least resistance to any residual stress, and so they expand the most. Constrictions also commonly appear where sight dovetails are cut into a barrel by a machining operation done with a dull cutter or a cutter with too high a feed speed. The other really common place to see constrictions are in revolver barrels where the tight threads screw into the frame.
Once you have tight spots, you have two options. You can hand lap or firelap the barrel to remove them. The principle is to embed abrasive in a lead lap and pass it through the bore. The constrictions will be abraded most, while the loose spots will be abraded least because the lap will not spring back out to bear against them after passing through a constriction. With a hand lap, you often just cast some pure lead directly in the bore and onto the end of a rod. After tapping it out of the bore you cut grooves in it to hold abrasive compound. You then load it up with the compound, usually about 320 grit, and work it back and forth. When it stops rubbing even the constrictions, you run the lap to the breech end of the rifling and use a dowel and mallet to bump it back up in diameter and repeat the stroking. You keep repeating this, adding abrasive if needed, until the barrel is uniform and free of constrictions.
Firelapping is easier that all that casting and cutting and the skill of constantly feeling for rubbing. It has its own problems, however. For one thing, you can't use those hard springy alloy bullets or you will find the cutting pressure is about equal in the constrictions and in the wide spots. That will polish but it won't remove constrictions. On the other hand, you can't use pure lead bullets either. Pure lead gets bumped up by the firing pressure after passing through a constriction. This doesn't happen in hand lapping and the result is the pure lead under pressure will polish in much the same way a hard bullet does. As a result, it is necessary to use a compromise: A bullet that is soft enough not to spring back much, but is still hard enough to resist obturating pressure at the lapping load level. Several different combinations of load pressure and bullet hardness have been proposed, but, generally speaking, a consensus has developed that you need a bullet that has a hardness of about BHN 11, fired with a peak pressure of about 10-15,000 PSI. BHN 11 does not spring back nearly as much as a BHN 16 bullet and will abrade harder, therefore on the constrictions than on the wide spots.
You can also firelap with jacketed bullets if you choose them carefully. I have done this with bullets pulled from M2 ball. These bullets come out with a crimp indentation in the center of the bearing surface so they only touch the rifling in two rings at the front and back of the bearing surface. Those narrow bands engrave completely in the bullet even with modest pressure, like 10,000 PSI.
I have also tried long bearing surface bullets like the Tubb Final Finsh system uses, thinking they would make the job go faster. But they did not fill into the corners of the rifling well at low pressure. Instead, as Tubb's instructions state, they have to be propelled by starting loads at the bottom of the normal load pressure range. Pehaps 40,000 PSI for most rifles. That is much higher pressure than is normally used in firelapping and is enough to bump these long bullets up to fill out the bore properly. Unfortunately that bump-up also means the bullet works the whole surface pretty evenly, same as an overly soft one would, removing the bore's wide spots as fast as the constrictions. That is why I say the Tubb system polishes rather than laps the bore.
Unlike hand lapping, because firelapping, in effect, strokes the lap in just one direction and because obturating pressure drops with the bullets travel down the bore, it will apply more lapping force near the breech than at the muzzle. Thus firelapping tends to taper the bore slightly narrower at the muzzle, where hand lapping tends to just make it straight. This is considered favorable for shooting lead bullets. With jacketed bullets, because they are under highrt pressure, obturate enough to not care much one way or the other. Indeed, some European shooters like guns with bores a thousandth or two over bullet diameter and use them very successfully with jacketed bullets. Hand lapping has the limitation that it does nothing to remove toolmarks from the throat of the barrel.
One of the keys to successfully firelapping a barrel is to constantly monitor progress. I run a new slug every 5 firelapping rounds, per NECO's instructions, which I find to be pretty good. Once constrictions are cleared, only polishing rounds are used. Firelapping M1 Garand barrels to clear their constriction under the thick part of their contour below the lower band purchase, I found about 20 of the pulled M2 bullets were needed when embedded with NECO's lab grade 240 grit abrasive. At that point the slugs no longer felt the constriction, and I switched to their finer polishing grades. At the end of the process it took 1/6 the number of patches and strokes with JB to get the barrel completely clean than it had taken before firelapping. The throat wear gage said the throat had been moved forward 0.001”. Not enough to matter.
Firelapping always makes a gun easier to clean. It does not always affect accuracy. Once in a long while it can even make a gun worse, though I would like to be able to study those cases because I suspect the process was done incorrectly using overly coarse abrasives, such as provided in the Wheeler kit, or because there was another problem such as crown irregularities that were exacerbated by the lapping process, or because excessive propellant was used. It is also logical to assume a gun would need its accuracy loads worked back up once the removal of constrictions and reduction in throat friction had occurred. These could easily affect barrel time and take the gun off a sweet spot.
A quick note on NECO's lab grade abrasives. These are not cheap but have two advantages. If you look at standard grade abrasives the particle distribution is pretty wide. The coarsest particle allowed is about twice the diameter of the average particle. The lab grade stuff has a much narrower distribution. This accomplishes two things: One is the chance a large particle will gouge a scratch deeper than average is reduced. The other is that fewer fine particles pack around the average size ones, so these abrasives cut faster. Amateur telescope makers report the cut glass about twice as fast as standard grades. NECO's coarsest abrasive is 240 grit lab grade, which means its largest particle size is about the same as a standard grade 280 grit particle would have. The Wheeler kit I caution againsts uses 220 grit standard grade. Its largest particle will be about the size of the height of a rifling land. That is too big. Precision shooting had a report by a fellow who used that kit on a .22 LR. It opened the bore half a thousandth, about 5 times more than the NECO kit opens a muzzle. That .22 shot more poorly after lapping. I'm not surprised, given that much material removal.
Nick
