Choosing and Using Precision Measuring Instruments

During my early training as a toolmaker, I quickly learned the value of having accurate measuring tools. Getting the hang of how to use these instruments afforded me a means to measure my progress when fitting the various parts during the fitting and assembly of functional gauges and holding fixtures. As I moved on into gunsmithing, I found these same tools also became invaluable in the fitting of many firearm parts.

Micrometers If there’s one measuring instrument anyone working on firearms should have near their bench, it’s a micrometer. A 0-to-1-inch micrometer will fit most of the gunsmith’s needs quite handily. A 1-to-2-inch micrometer gets occasional use, but is not needed as frequently as the 1-inch. The micrometer has two anvils. The front anvil is stationary while the rear anvil moves when the increment barrel is turned along the lead screw. Micrometers have a 40-thread-perinch lead screw.

Thus, with a full rotation of the barrel, the movable anvil is advanced .025 inch. Along the barrel are corresponding dash marks that are calibrated in perpendicular spaces equal to one-thousandth of an inch. With some micrometers, the barrel that houses the advance spindle has an even finer measurement bar. This breaks your measurements down into ten-thousandths (.0001) of an inch.

During use, the part to be measured rests against the stationary anvil, and the micrometer thimble The vernier-type requires aligning witness marks to read measurement. is then turned toward the part until contact is made. When measuring round parts, to get an accurate measurement it’s best to rotate the movable rear anvil of the micrometer slightly to find the high point opposite the stationary anvil. Most micrometers are equipped with either a slip thimble or ratcheting device at the rear of the thimble.

When the thimble begins to slip or the ratchet clicks, you’ve reached the point of reading your measurement. This helps prevent any over-tightening of the measurement thimble that could record a false reading. There are three styles of micrometers available: the vernier style, where you need to align witness marks on

the barrel to the recording thimble; the analog type that has actual numbers that click into place as you turn the thimble; and the digital style that is the easiest to read, but also the most costly. A 0-to1- inch micrometer will cost anywhere from about $20 to nearly $150, depending on the style and brand you prefer. A depth micrometer has interchangeable stems in 1-inch-long increments, with most sets measuring 1 to 6 inches.

This style of micrometer has a flat base that rests against a shoulder and records the distance from that shoulder to a targeted face or shoulder. A couple of examples would be the distance from the front face of a receiver on a bolt-action rifle (with the barrel removed) to the front face of the bolt. This measurement is needed to establish the barrel-shank length. Another example would be the measurement from the front face of a revolver frame to the cylinder face to establish barrel-to-cylinder gap.

During a rebarreling and threading job, the depth micrometer will also help to get the correct length of the barrel shank by measuring from the rear barrel face to the barrel shoulder. When installing front sights, the depth mike is used to help get the sight centered in the sight base. By measuring each side of the sight to the ramp, you’ll know exactly how far the bead insert will need to be moved to become centered in the ramp dovetail.

One type of micrometer that gets limited use, but is nonetheless useful, is the internal-groove-type micrometer. The one I have is made by Starrett, and comes into play most often on semi-auto pistol slides to measure the right-to-left distance of grooves when fitting slides to frame rails. This type of micrometer will reach down into areas that are difficult to get at with other measuring tools. Calipers Another measurement tool that’s useful to have around is the caliper.

A vernier-type caliper is something like a slide-rule, if you remember how those operate. These are low-cost measuring instruments, and require the reading of witness lines that align when the caliper jaws are drawn together over the part being measured. While not as accurate as a micrometer, a caliper will do for 90 percent of the measuring done in gunsmithing. The next level up would be the dial caliper. This style has a geared dial that rides along a tooth rack in the caliper.

A dial with a recording needle lets you know which increment between the witness marks along the caliper you’ve reached for measurement. The calipers that are easiest to use and read are those with a digital readout. These calipers have a built-in scale reader that displays your measurements on an LCD screen incorpo Below top: The vernier-style caliper requires the alignment of lines to achieve a precise measurement. Below bottom: The dial caliper shows its measurements on an easy-to-read dial.

rated into the movable rear jaw of the caliper. If you’re ready to spring for the cost of a caliper, this would be the type to go with. The cost of this type of caliper will depend on whether you choose an established name brand or one of the many imports that are coming into the country. A Starrett, Mitutoyo, or Brown & Sharpe digital caliper will run close to $150, while an import will typically cost from $30 to $90.

These calipers are usually available in 4-, 6-, and 8-inch lengths, with the 6-inch being the most popular for our type of work. The caliper will not only measure outside diameters or distances, but also has blades that will record inside diameters and distances. A depth-recording stem is also provided that moves out the rear of the caliper as the movable jaw is retracted.

If you happen to be a reloader of metallic ammunition, the caliper will also find use on your reloading bench to measure the trim length on cartridge cases after residing and turning. Indicators If you’ve ever operated a metal lathe or milling machine, you know the value of a dial indicator. Dial indicators come in quite a variety of styles, and are available in both the dial and electronic (digital) variety. These instruments will record in .0001, .0005, and .001-inch increments, depending on your accuracy needs.

For most gun work, indicators recording in .001inch will do just fine. (You’ll find that indicators reading in the .0001-inch range usually have limited travel, so the recording range will not be as great as the .001-inch style.) Dial indicators are invaluable for finding out how much run-out you have in apart, and where the high points of that run-out are. A couple of examples would be a bent ejector rod on a revolver cylinder or a bent firing pin.

Place the indicator tip over top dead center of the firing pin while it rests in a V-block, and rotate the bent part one full revolution to see the amount of bend this part has. An indicator mounted on a surface-gauge base will tell you how flat apart is. An example of this use would be on the flat bottom of a rifle action to determine if the action was twisted during barrel removal. I have a dial indicator that will record 2 full inches of travel in .001inch increments.

This indicator has saved my bacon many times when drilling blind holes to a specific depth in thin-walled material. Every time I need to drill ramp-mounting holes on the muzzle end of a thin barrel, I use an indicator to record the depth the drill is cutting. The indicator is set against a collar mounted on the drill’s quill, and as the quill is fed down against the indicator tip, the recording needle shows exactly how deep the drill is cutting.

Conclusions There are many imported measuring tools on the market, some of which are worth the money and some best avoided. Going with the brands that have been around quite awhile— Starrett, Mitutoyo, Brown & Sharpe, or Fowler—you’ll never go wrong. I know that some of the imported gauges sell for less than the cost of a box of .30-06 ammo, and I’ve looked some of these instruments over carefully.

My choice has been to go with a well-known American company that will be around should something go wrong with the gauge and I need to return it for repair. ■