Lapping Barrels And Bore Polishing

"|T APPING is an operation involving the cutting of a piece of ^ metal by rubbing it with a softer piece of metal coated with an abrasive. The operation of grinding automobile valves is not lapping, but grinding; for both the valves and the block are of similar hardness, and both are cut by the valve grinding compound. If the valve were made of lead or soft brass, the grit would bed itself in the metal, actually cutting it very little, but would cut the valve seat even faster for that reason.

Lapping a rifle or a pistol barrel involves the casting of a soft lead slug in the barrel; and this slug, when coated with fine abrasive, really becomes an abrading tool, accurately fitted to the bore, and because of its softness and the manner in which the Another way that is very effective when carefully done is as fol- abrasive clings to it without cutting, the lap wears much more lows: First scour the inside of case bright with emery cloth wrapped slowly than the harder metal of the barrel.

on a small stick. Be careful not to let the emery touch any part of Most rifle barrels are improved by careful lapping. This is not the chamber or bore. Push a small cork or wooden plug well down saying, • however, that any finished barrel can be thus improved, into the neck of cartridge case, the end of plug projecting about 1/16 Because accuracy is to a large extent dependent upon correct bore

Lapping Bores
Fig. 163

diameter and proper bullet fit, and the factory barrel that has been finished to maximum bore size without lapping, might be so enlarged by lapping later on that accuracy would be ruined. The barrel that in process of manufacture is brought to its final diameter by slow, careful lapping, is likely to be a more accurate barrel than one that is not lapped. Having a factory made barrel that fits the bullets tightly, and which does not approach maximum dimensions too closely, one may often improve its shooting qualities materially by lapping just enough to smooth out any faint traces of tool marks, and to remove any tight spots such as are often encountered. But the job must be approached advisedly, and only after careful bore measurements have been taken, as outlined in Chapter 16. 390

Even a barrel that runs close to maximum bore diameter may be improved by lapping, provided there are cross reamer marks on the lands—and provided the lapping is carried no further than the removal of these marks, which often result in leading or metal fouling. Again, an old barrel that was tight when first made, may be somewhat worn or pitted from rust; and while lapping the bore smooth will result in this instance in considerable enlargement, careful selection of a tighter fitting bullet will give surprisingly better results. Even an oversize bore may sometimes be lapped out larger and accuracy increased by the use of larger bullets. For example, one may have a Krag that is inaccurate and largely useless by reason of a bore measuring .3095 groove diameter instead of .308 as it should be. By lapping it to .3105 or .311, the .303 British or similar bullets may be used with excellent results, and a rifle that was formerly useless may be made serviceable and accurate. In addition to factory bullets, one may use cast bullets sized to the larger diameter required, or even use them as cast, without sizing, in some instances. The Squib-Miller bullet, for example, will give excellent results in sub-loads in such a barrel, without sizing, and so will the Squibb gas-check bullet.

The equipment needed for barrel lapping consists of a lapping-rod, a small iron pot for melting lead, an Ideal or Bond dipper for pouring, some cotton string or waste, some good light oil, a few jap slightly at p^ The improssion of the lands and grooves ounces of the very finest grade emery flour obtainable also some shouW bc full md $harpt whh yery {cw ^ ^^ fine powdered pumice, fine crocus powder, and powdered rotten NoWj lcaving the lap ¡n thc barrel( ^ barrel horizontally in stone. Most of these materials will already be on hand m Hie ^ visc> ^¡„g brasa> lcatbcr> or jclt paddcd Jaws ^ prcvcnt ^^

amateur gunsmith s workshop. # barrel push lap ^^ two-thirds of its length out of the muzzle,

Figure 164 shows details of a lapping rod which is splendidly and coat it lightly with emery flour mixed with oil. Use lots of adapted to the work. The handle assembly involves considerable oi\ and very littic cmerv at See that the vise is firm and machine work in the making of thc ball bearing ways, and this may ngl(J> Grasp the handle and draw rod toward breech with a be avoided by buying a bicycle front hub and axle complete and stcady continuous motion, until the wrapping of string is exposed, brazing or welding the lapping rod to the end of the axle. A ^ tbc string should be removed at once. Withdraw the lap partly handle is then made of wood or metal and attached to the outer from breech and coat the rear end also with oil and emery flour, shell of the hub by clamp screws, or by brazing or welding if a metal Attach a block of wood well padded with rags about an inch ahead handle is used. A handle made of 3/8" cold rolled rod, bent round of thc muzz\tt to act as a stop and prevent the lap being pushed the hub and brazed to it is perfectly satisfactory, and is made more out the n^zle. Then proceed with the lapping, by grasping the comfortable to use by puLing a short piece of thick rubber tubing handlc of rod firmly ¡n foth hands, and pushing it back and foCth over each end. Rubber milking machine tubing 1$ excellent for with barrel length strokes. The lap should project from the this purpose, as it is thick and soft. The handle must be large muzzlc t0 8trikc th< stop cach tirac ^ should also be partly enough to grasp with both hands, as considerable force is often withdrawn from the breech on every back stroke. But at no time necessary in using the rod. musi t\€ \&p fo entirely withdrawn from the bcre, and if accidentally

Clean thc barrel thoroughly, using ammonia dope to remove any withdrawn, dip it in the melting pot and melt it off, replace the metal fouling, and a wire brush, mercury, or whatever proves rod ¡n barrel as in the beginning and cast a new lap.

necessary to remove leading. Every particle of foreign substance Rjfle bores are like fingerprints and women—no two are exactly must be removed. Examine the barrel carefully from both ends aiifcc. Moreover, no two lands or grooves in the same barrel are and mark with chalk on the outside the position of- any tight and exactly alike; and while a lap may be replaced to apparently a loose places, rough spots, pits, reamer marks, etc. Measure the pcrfect fit, be assured it does not fit,'and the corner of one or more

391 lands is likely to be rounded, or the botrom of a groove made uneven, barrel with a lead slug as previously described, noting the bore x^, warning may bring forth a smile of derision from certain and groove diameters carefully, and deciding on the maximum barrei m^eTSt wh0, when nobody is Iootang, are in the habit of diameters permissible after lapping. After cleaning, swab thc ^fog Dnc jap ¡n tbrcc or four or half a dozen barrels, inserting barrel with a good light oil, then swab dry with clean patches. ¡t by the feel of the lands, and giving the barrel "a lick and a promise*

Wrap a bit of cotton string or waste around the lapping rod in the belief that thc purchaser will never know thc difference any-

just back of the tapered jagged tip in the cannelures. Insert rod how. These fellows may consider such methods as good shop through barrel from breech, so that tip is an inch or so below the practice, but I don't. In my opinion it's a jackleg system, does muzzle. Warm the muzzle over a gas burner until it is about as no good, often does harm, and it's plain dishonest That's my hot as you would have a bullet mold for casting, then quickly set story and I'm going to stick to it.

barrel muzzle up in vise and pour in melted lead even with muzzle. Flour of emery cuts rather fast, and the bore should bc carefully

This prevents excessive shrinkage and gives a good full lap. When watched so as not to lap it too large. However, one is perfectly the lead has cooled somewhat, push the lap partway out of muzzle, safe in re-coating the lap with emery and oil three or four times and file off the fin of lead where it ran over the muzzle, tapering the before it is removed for measurement. Then carefully flush out

Expedient Shotgun

the bore with kerosene, mop out with a loose patch, then "with patches wet with thin gun oil, then dry. Be sure every particle of emery is removed, then measure the bore with a lead slug to ascertain how much the first lapping has enlarged it. If it is desired merely to smooth up the bore, probably one lap will be sufficient.

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    5 years ago
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