Making and Using a Locking-Lug/ Bolt-Face Comparator

Many times, someone has come into my shop, seen my locking-lug/bolt-face comparator, and asked: "What in the world is that thing?" No one who’s ventured a guess has ever come close. Necessity being the mother of invention, some 30 years ago I invented a tool to determine the precise length between a bolt face and its locking-lug camming surfaces. Initially, I used this compared measurement in my handloading. It told me if I could interchange brass cases from one rifle to another of the same model.

It also provided me with information when truing locking lugs. Still another use where it proves invaluable is when using the same barrel in different rifle actions. I work with a number of switch-barrel rifles, and commonly use the same barrel in more than one action. This is particularly likely to occur with my Remington Model 700 and Model 40X actions, where I may have five or more barrels fitted to one or more actions.

A recent situation where the comparator came in handy was fitting and headspacing a Douglas barrel in 6.5-06 caliber to both a 40X and 700 Remington action for this cartridge. Before undertaking the chambering and headspacing, the comparator told me which of the rifles I had in mind were compatible. After chambering was completed and load testing got underway, I discovered I could actually interchange fired cases from one action to the other. The resultant accuracy was excellent.

Making the Comparator As the photos show, this little arbor-press-looking gadget is quite uncomplicated. From a 1-3/4- x 1-3/4- x 5/8-inch block of aluminum or steel, cut a 1- x 3/4-inch rectangular window just below center. The 1-inch width of the opening will accommodate the maximum outer width of most bolt locking lugs. With the window cut out and perfectly squared, cut an opening at the bottom 11/16 inch in width. The bolt body fits this opening.

With the block shaped and finished to the described dimensions, drill and tap a 1/4-28 hole centered through the top of the unit. Be sure this hole is absolutely square with the block and locking-lug jaws. Next take a fully threaded 1/4-28 x 2-5/8-inch bolt and square the head to a 5/16-inch width. Leave a remaining base about 1/16-inch thick as a contact surface to fit the rifle’s bolt face. Attach a double nut at the top of the bolt used as a lock knob to turn the threaded ram.

Attach a pointer to the bottom nut which will later be used to indicate its position on the scale. As a scale, I stamped numbers and dots in 1/1000-inch increments The two bolts are from a Remington Model 700 short-action and a Remington Model 40X.

The dial is calibrated in 1/1000-inch increments. around a flat washer with a 1/4-inch hole in the center. This rotary scale is held in the desired position with a socket-head screw tapped into the top of the gauge. The scale is calibrated in increments of 1/1000 inch, which is 35.7 (36 is fine) equidistant markings around the periphery of the scale.

Putting the Comparator to Use To use the locking-lug/bolt-face comparator, slip the jaws of the tool against the locking lugs with the bolt-face surface centered on the face of the ram. Hold the instrument tightly against the locking lugs as the threaded ram is turned to make contact with the bolt face. If held at true center, the flat surface of the ram will push a Remington ejector pin inward against its spring tension. At this point, turn and lock the scale at zero with the pointer.

It may be ad From the visable to make a few measurements to be sure of repeatability. Measurements can be taken of other bolts for comparison. The locking lugs of the bolt will self-align as the flat surface of the threaded ram makes firm contact with the bolt face. The comparator can be depended on to repeat readings with an accuracy of 1/1000-inch or less if all parts are held in alignment.

To make a reading of the space between the locking lug surfaces and the bolt face, remove the bolt from the unit, reset, and measure the span between the two jaws of the comparator and the flat surface of the ram. Be sure the ram is returned exactly to the recorded scale position before taking the reading. Conclusion Determining the length between a bolt face and its opposing locking-lug surfaces can be a useful measurement for the gunsmith or anyone who needs an exact comparison between two bolts.

Making such a comparator is an uncomplicated machine-shop process, and cost is minimal. ■