Unilateral Locking Lug Lapping

Serious accuracy devotees use many things that may enhance rifle precision, including blueprinting the entire rifle action. However, in many cases this entire process may not be necessary. One area of concern in my rifle precision evaluation does include uniformity of bolt locking lug contact. It is easy to check out and quite easy to correct, if needed, as I shall outline in the following paragraphs. A simple visual examination for locking lug surfaces may reveal inherent problems in this area.

Close inspection of the working surfaces of each lug will tell the story. One lug is likely to reveal more surface contact than the other. This will be the lug with greater bearing surface. A good way to inspect the lug surfaces is to apply a coating on each lug with a magic marker or other means of darkening the surface. Working the bolt on a firmly chambered cartridge or headspace gauge will show contact differences.

This may require thin shims between the bolt face and cartridge as the bolt is worked several times. The result will be a noticeable removal of blackening and show a shiny lug surface. The vernier caliper shows the diameter of the bolt at 0.695, the same as the nylon tube. Bottom The tube must first be split lengthwise for use in lapping the lugs.

Where one lug shows greater contact than the laterally opposing one, I have developed a means of reducing the bearing surface of the long lug to equalize contact surface between the two. This procedure must be done with precise coaxial congruity with the bolt body. The pictures will show details of how this is performed. To make up a lap I began with a one-inch diameter nylon tube three inches long. The tube has a hole diameter the size of the Remington bolt.

The tube was faced at each end in a lathe, cut lengthwise down the middle and the burrs removed. I happened to have apiece of tubing with

The measurement will show the difference in length between the bolt face and locking lugs of a rifle bolt. the correct diameters for the Remington bolt, which saved drilling and boring it. During use, one end of the sleeve is coated with a fine abrasive (about 200 grit) which makes contact with the longer bolt lug as it is rotated. The amount of applied pressure against the bolt lug would be about 10-20 pounds as the sleeve is rotated in half circles back and forth.

Normally the difference in contact between the short and long lugs will be but one to two thousandths of an inch (0.001- 0.002 inches) so move slowly and check your work. Clean up the lugs, re-apply blackening and try the bolt with a fair amount of force. Use apiece of paper between bolt face and case head if needed. Writing paper or computer paper is about 0.003 inches and is compressible as the bolt is closed. The object is to remove only as much metal as needed to equalize contact between bolt lugs.

Plan to stop as soon as this amount of metal is removed from the longer lug. This shouldn’t noticeably affect or alter headspace enough to require barrel shoulder changes. At the onset of this procedure I use a special tool I designed called a bolt face comparator. The tool precisely compares the difference in length between locking lug surfaces and the bolt face.

I use this tool to measure bolts for my switchbarrel rifles and where I may want to interchange bolts between rifles while maintaining the same headspace. Over the years I have found that the Models 700/40X bolts are extremely uniform as headspace is compared. The tool works on any rifle bolt that has two laterally opposing lugs. A brief point of discussion involves barrel shoulder and bolt face axial preload and how it affects cartridge headspace.

As a rifle barrel is turned into position with the barrel shoulder forced against the receiver face, this is termed axial preload. Such loading may be well over 10,000 lbs. with barrel torque of 150 to about 250 foot pounds. Thread pitch as well as thread lubricant used make a significant difference here. The degree of axial pre-

load has a definite effect on varying cartridge headspace ranging from approximately 0.001 to as much as 0.005 inches in extreme cases. For my many switch barrel rifles I apply about 75-100 foot pounds of torque as I change barrels, about the amount of torque used to tighten a lug nut as you change a tire. Bolt face thrust can be well over a thousand pounds against a case head which affects axial preload to some degree too. Small operations can play a major part.

Accurizing a rifle can involve many different steps, which does include uniform locking lug contact. Not all these steps involve a major costly process as this simple and effective way of unilateral lapping of a bolt locking lug has proved. AG A small amount is actually spilled over onto the bolt lug. Bottom The opposite lug is now showing contact as well nearing precision bilateral contact. 001 inch of metal was removed to make uniform contact between the two locking lugs.

This is a very convenient and worthwhile procedure in arriving at uniform bilateral bolt lug contact.