Barrel Bore Lapping

Rifle builders and shooters alike must understand that bullets propelled by burning powder gas leave carbon and bullet jacket deposits in the bore and also cause bore erosion. Repeated firing exacerbates the condition. The rougher the bore, the more difficult these accuracy inhibiting conditions will be to prevent or correct. In most cases, a well-established form of bore lapping may be the best approach to the problem.

This may include conditioning anew rifle bore that has not been lapped, particularly those of lesser quality, or bores with some use. In any case, a knowledge of bore condition must precede any attempts toward corrective measures. The premise behind lapping is to dress smoothly the bore metal sur- We are seeing a pretty good looking bore finish on this production rifle here. faces to a high degree of refinement and accuracy, thereby allowing a more uniform bullet passage up the bore.

A rudimentary familiarization of the mechanics involved in bore production should precede any form of lapping or bore breakin attempts. The final condition of a bore may be dependent on such understanding. Following deep drilling, a barrel is reamed to form the surface of the lands. This smaller diameter will comprise the inside surface diameter of the bore as the bottom surface of the lands or the resultant depth of the rifling engraved on a bullet.

Following this deep drilling and reaming, the rifling grooves will be formed, resulting in the groove diameter essentially the same diameter as the bullet. This is typically expressed as 0.256/0.264 or 0.300/0.308 for 6.5mm and .30 caliber bore/groove diameters. The grooves are formed by varying processes which typically include button rifling, cut rifling, broaching or hammer forging. This leaves the overall diameter of the grooves very close to bullet diameter.

As evidence of this work, bores are left with residual machining marks incurred as a result of the reaming and rifling processes. Such barrels may be delivered to the shooter or gunsmith as rifles or barrel blanks with a wide range of smoothness or finish. The better barrel makers produce a high quality, essentially finished bore usually requiring minimal break-in for best shooting. Most of the others may require varying degrees of attention in this area.

The better quality bores undergo a lap finish to extreme tolerances in diameter and uniformity as well as surface finish or refinement. But even these will require varying degrees of shooter break-in as a preventive measure of accuracy-robbing copper fouling. Bore Condition Internal bore surface evaluation may be performed in a number of ways. This may include the use of a bore scope, excessive cleaning brush drag, greenstained cleaning patches while using cop-

per removing solvent, or actual visual evidence of copper or bore roughness. Arguably, the modernday borescope will be our best aid in bore surface evaluation. The reputation of your barrel maker or firearms manufacturer is also very important. Having determined the state of the bore, the approach may vary in getting it smooth and shooting at its best. The better bores may only require a prescribed shoot/clean regimen, and others some form of lapping or dressing.

One should never dive in and try to better the lapping quality of those better bores. The method of lapping that I have used and will demonstrate here will lap and polish the outer surface of the lands where they have been reamed, as well as the grooves which may have been formed in different ways. Both can be accomplished with a special caliber specific lapping button of which I will describe.

Making Lead Laps All that is required in making these precision laps is some soft lead slugs and short pieces of 8-32 threaded bolt stock to fit a standard cleaning rod. Here I begin with an oversize flat-base lead slug, in the form of a bullet, and place it in a lathe chuck and drill and tap for a short 8-32 bolt. Thread the bolt in-line about 3/8" into the slug. These slugs are then sized according to the exact bore size.

I then place the screw lightly in the lathe chuck and use a sharp pointed tool bit to turn the laps to precise bore groove diameter as needed. An oval taper is formed at each end of the lap. This will aid in starting the lap into the bore as well as retaining some of the lapping medium for distribution. Annular rings on the lap will also retain lap abrasive as needed. I make up several of these caliber specific lead laps.

For lapping of the grooves and lands, the lap will need to be of bullet size for that bore or about one thousandths over. These slugs can be easily turned to size in a lathe. Be very precise about this. Some actually melt the lead lap into the bore. Whatever works for others; I have no quarrel with this. Forms of fire lapping are also used. Some of the barrel makers void the warranty in instances where firelapping is used.

As a perennial research-minded experimenter I have developed a method of "shoot lapping" whereby the bore is anointed with a viscous lapping compound and the bullet is shot over it, thereby lapping the entire bore. This has received some favorable acclaim by some good names and I may cover it in American Gunsmith at a later date. But for now the threaded slug will be the subject of this treatise.

Bore Lapping Abrasives Many chapters could be written on this subject alone but I will keep it as concise and meaningful as possible. My association with one of the best metallurgists and rifle experts in the business, Mr. Fred Barker, has been of much help to me on selection and use of abrasives. We both were staff writers for Precision Shooting Magazine until its closure. A lapping medium is essentially an This technique is inherently uncomplicated and effective as a means of bore lapping.

The bore is then ready for a prescribed method of shoot-and-clean bore break-in. This is sometimes performed via fire lapping where a grit-impregnated bullet is used, but not in my rifles. abrasive used to dress or polish a roughened surface. Such lapping mediums are composed of varying degrees of abrasion which must be well understood and used correctly. In lapping I seldom use a grit courser than 400 U. S. Mesh, which is 30 Micron grade.

A good choice here is J-B Non-Embedding Bore Cleaning Compound, a non-embedding, pumice-type abrasive that will readily size or lap bore metal. Evidence of the use of grit is seen as the stria pattern in a professionally-lapped rifle barrel. The choice of lapping medium is often a thing of experience and is learned. Lapping compounds can be purchased, but be sure of grit size. Never use a grit that is too coarse as it can score the metal.

With a correctly selected lapping button for the bore being lapped, attach it to the cleaning rod or shorter shafted starting mandrel. Screw it on firmly so it won’t turn off. Then, with the barrel held horizontally in a vise (preferably removed from the action), the abrasive coated lap is passed forth and back through the bore. Always fully complete each pass, but not fully through the muzzle.

The first passage of the lap will actually show grooves cut into it by the rifling lands, offering a fairly firm resistance. You may need to tap the cleaning rod or starting mandrel with a plastic mallet to get it going. Then, while carefully guiding the rod, push it into the bore to the end, but not through. You may feel varying resistance in different sections of the bore due to bore diameter difference or bore surface smoothness.

Pass the lap fully forth and back a few times to get a feel for things before applying additional lapping medium, but don’t use too much compound—a little goes along way. A cleaning rod guide is recommended, or minimally, a cham-

The four groups at left were shot pre-lapping with a 30BR MAG, a cartridge designed by author by necking the .25 Winchester Super Short Magnum case to .30 caliber. The four groups on right were shot post-lapping with the same load of a 150 grain Berger FB Target bullet. Lapping and bore break-in was conducted between groups over a week-long period. bered drilled-out cartridge to help guide things along. Never allow the cleaning rod to contact the muzzle of the bore.

As the button wears down, install another one following the same procedure. A practice session on an old barrel may prove helpful for some. Typically the bore will take on a noticeable smoothness permitting easier passage of the lead lap. Careful examination with a bore scope should show things much more clearly. But be advised, no matter how smooth and shiny the bore appears following lapping, it still must go through a well-established shootclean, break-in regimen on its way to its best shooting.

Bore friction is by no means an inconsequential factor in our shooting and must be recognized. For example, a 50% reduction in overall bore friction allows an increase in velocity of about 10%. Further extrapolated, this has the potential for an additional 175 foot-pounds of muzzle energy. Of course, this depends on bullet configuration and coefficient of friction, which is a study in itself. In my own experience, accuracy is the greater derivative of lapping.

There is a great deal of learning and experience that must be applied in our quest for better shooting. Bore lapping and barrel break-in are but a single link in the chain. AG