Lap Fitting of Rifle-Bolt Locking Lugs

In blueprinting and accurizing a bolt-action rifle, particular attention is always given to inspecting the receiver, and corrections are routinely made where needed. The locking-lug surface contact must not be overlooked in this process, since the symmetry of the bolt’s locking-lug surfaces plays a vital part in the accuracy chain. In accurizing many bolt-action rifles, I’ve found that careful attention to the locking lugs alone can correct accuracy problems in some cases.

A good number of do-it-yourselfers don’t follow the whole blueprinting plan in their quest for better accuracy from a rifle, and failure to pay attention to the locking lugs can compromise an otherwise good accurizing job. The procedure shown here is both simple and effective. Moreover, it can be used by both the do-it-yourselfer and the professional gunsmith with a minimum of cost and time.

Basic Bolt Inspection Once a rifle has been fired several times, it’s easy to inspect the lockinglug surfaces for contact uniformity. Metal abrasion usually is manifest as a shiny surface, usually seen more on one side than the other. Where normal firing of the rifle fails to reveal an asymmetry between surfaces, Prussian Blue or a permanent felt-tip marking pen can be used to show points of wear.

I apply a coat on both bolt-lug surfaces and close the bolt on a fired cartridge, which will chamber with some resistance resulting in axial pressure on the lug and receiver mating surfaces. Then carefully inspect each surface for contact differences. Normally, the 6:00 o’clock (lower) locking lug receives the greater wear, which is caused partially by the action of the bolt-cocking mechanism. This depends also on how well the bolt fits the receiver.

Where a fired case is not available, simply perform this step by closing the bolt and pulling firmly rearward on the bolt handle while working it up and down. Where there is no feeling of resistance as the bolt handle is moved up and down, pulling firmly rearward on it will apply some pressure. Another means of applying rearward axial pressure, both in testing and lapping, can be applied by placing a circular disk of shim stock between the bolt face and the cartridge.

Trying In all cases, however, the symmetry of the locking lugs is an important factor in the accuracy equation. different thicknesses will readily show which one works best. A sheet or two of paper will also work here. Lapping the Bolt Lug Surface Lapping the bolt lug surfaces is a very simple operation. Just place an abrasive material on the lug that shows the most contact, and close the bolt fully. This will deposit lapping material against the proper surfaces.

Again, using a fired cartridge works well, but isn’t absolutely necessary. With the bolt in place, work the handle up and down a number of times and then remove the bolt. Lightly wipe down all surfaces and reinspect for contact

Above center: The striations on both lugs shows uniform contact side-to-side, which means excellent bilateral locking-lug contact. Uneven contact here can mean trouble. uniformity. What we’re trying to do is to achieve uniform contact bilaterally, and lapping should be stopped as soon as the lugs show even contact. Usually there is little contact difference and the procedure will not take long.

The shim-stock disk can be applied during the lapping procedure, but be sure to avoid getting any lapping material on the shim or the boltface. In fact, a drop or two of oil here is a good idea. As a lapping medium, I use a very fine homogeneous grit like 400 or 600 aluminum oxide or silicon carbide, or even something as fine as J. B. Bore Paste sold by Brownells. A paste-type medium is usually easier to work with than an abrasive powder mixed into oil or grease, but either will do the job.

As the lapping operation is performed, be conscious of the fact that you’re not trying to appreciably alter the head space of the rifle, and it is rare where more than a thousandth or two of contact difference between the two lags will be noted. If it appears to take more than this, I would normally Both the face of the receiver and the locking lugs must have uniform contact. The J-B Bore Cleaning Compound has sufficient abrasion to lap the locking lugs as described in the text.

Extreme care is taken in this procedure while using a micrometer lathe carriage stop. stop the procedure and leave well enough alone. You will quickly note the progress as you work along unless it’s a stubborn situation, in which case a more involved means of correction will be required with barrel removal and set back. Conclusion In the never-ending search for accuracy in a rifle, looking to locking-lug contact is a good place to start.

This easy method of attaining more uniform lug contact requires only a basic inspection and simple corrective measures that are easy to perform. I

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