Measurement of bore and groove diameters

In a barrel having an even number of lands and grooves the bore diameter is the distance between the tops of opposite lands. In a barrel having an odd number of lands and grooves the bore diameter is the diameter of a circle which would touch the tops of all of the lands. The groove diameter for a barrel having an even number of grooves is the distance from the bottom of a groove to the bottom of the opposite groove. The groove diameter for a barrel having an odd number of grooves may be defined in two ways. It may be defined as: (A) The diameter of a circle which would just touch the bottoms of all of the grooves or (B) The distance from the bottom of a groove to the top of the opposite land. The second definition has been chosen for the measurements recorded in this work because the distance from the bottom of a groove to the top of the opposite land is readily measureable.

The conventional Starrett Small Hole Gauges can be used for measurements of the land-to-land (i.e., bore) diameter for barrels that have an even number of lands, but they cannot be used for the measurement of groove diameters because the gauge will not reach the bottom of the groove. (The „split ball" of the gauge will strike the „shoulders" of the groove, except possibly in some cases of large-caliber barrels having exceptionally wide grooves.) And these gauges cannot be used to measure either groove or bore diameters where the barrel has an uneven number of lands and grooves.

The problem of making satisfactory measurements in all guns was solved in the following way. Three sets of Starrett Small Hole Gauges were procured. One of these sets was used in the conventional manner for measuring land-to-land diameters. A second set was provided with a pair of oppositely placed „fins," as shown in Figs. 89 to 91, and was used to make the measurements of groove diameters for those barrels that had an even number of grooves. The third set (Figs. 89, 90, 92) was modified by providing a single „fin" on each gauge and these gauges were used to measure the „groove diameters" (the distance from the bottom of a groove to the top of the opposite land) in barrels having an odd number of grooves. These „fins" may be soldered on with silver solder, or, as pointed out by B. D. Munhall, they may be made by using a larger gauge and grinding away the unwanted metal until the desired shape is produced. The „fins" made by the second method will probably be more durable, but no particular difficulty has been experienced with the original method.

In the case of barrels having an odd number of lands there remains the problem of measuring the land or bore diameter, which cannot be done with the Starrett gauges. Consequently resort was had to the time-honored method of using tapered steel gauges to make these measurements (Fig. 93). These gauges are made of tempered, high-carbon steel and are accurately ground so that the taper is uniform. The over-all length is 3 inches and the tapered portion is about 23/8 inches in length. The difference in diameters at the ends of the ground portion is approximately 0.020 inch. This small change in diameter naturally increases the accuracy of the measurements. The entire set consists of 13 such tapered gauges, covering the range from ca. 0.200 inch to 0.465 inch. In using the tapered gauge, a bit of colored vaseline is placed on the gauge at the point where it will make contact; the gauge is then inserted into the barrel and is rotated gently as it makes contact. This leaves a colored ring with a sharp line of demarcation and the diameter of the gauge at this point is measured with a Starrett micrometer having a vernier enabling one to make readings to 0.0001 inch. (This micrometer is also used to measure the other gauges described above.) For convenience the micrometer is mounted on a stand, leaving both hands free to hold the gauge and to operate the micrometer (Fig. 94).

With the tapered gauge the measurement of bore diameter has to be made at the end of the barrel. This is unfortunate because the bore may not be the same at this point due to the

„crowning" process, to accidental burring, or to wear. In the case of the barrels with even numbers of lands, the diameter can be measured at any portion of the barrel that can be reached by the gauge and the measurements can be made with more confidence. Any nonuniformity (or inequalities) in either the bore diameter or groove diameter can be readily detected (and measured) in such barrels. In this way a very good idea can be obtained as to the perfection of the rifling, or the condition of the rifling if the gun has seen considerable use. This statement applies to the grooves in guns having an odd number of grooves (where the modified Starrett gauge can be used) but not to the land diameter since this has to be measured with the tapered gauge. In general it is good procedure to make measurements at least one inch from the end of the barrel because more uniformity is found there than at the muzzle. As with all such measurements, several settings should be made and the average of the readings taken.

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