An alternate method for measuring land impressions

A fairly satisfactory measurement of the width of a land impression on a fired bullet may be made by using a stage micrometer, graduated to read to 0.01 mm., in conjunction with a comparison microscope. The stage micrometer is a glass microscope slide, about 1 x 3 inches, upon which is engraved an accurate scale, in either the English or metric system, i.e., graduated to 0.001 inch or to 0.01 mm.

The bullet is placed on one stage of the comparison microscope and the micrometer slide is mounted on the other, with a bright white reflector underneath to bring out the lines of the scale clearly. The bullet must be carefully positioned (by rotating) until both of the edges of the groove are in focus. Final adjustment of the position of the micrometer slide is made so as to bring the zero of the scale to coincide with one edge of the groove and then the width of the groove can be read directly on the scale. Experiments were made with steel scales graduated to 0.01 inch but the desired accuracy could not be obtained because of the coarse graduations and the width of the engraved lines.

A comparison of this method with the method described above, and regularly used by the author, shows that values obtained by the two methods agree quite closely for bullets which show welldefined, clear-cut grooves. For bullets having less well marked land impressions the method involving stereoscopic vision is much better.

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