Measuring Impressions

Most general rifling characteristics can be readily observed using the naked eye. However, the determination of the dimensions (width) of land and groove impressions presents additional considerations.

These considerations include the following:

• The widths of the land and groove impressions are small and must be accurately measured.
• A discipline-wide technique must be used to ensure reproducibility of results to compare against a standard.

For measurement accuracy and reproducibility, reference points that are common to all bullets fired from conventionally rifled barrels must be established. These reference points define the location at which a land impression meets a groove impression. The width of the land impression is the width of the floor; the plateau and both adjacent slopes define the width of the groove impression.

Techniques

Measurements may be taken by a number of techniques. Regardless of the microscopic technique, the reference points must simultaneously be in focus; the slightly rounded surface between them may be slightly out of focus.

Land and groove impressions on damaged bullets may not be visible and/or measurable (useable). While it may not be necessary to take all measurements, it is important to take reliable measurements.

Examiners notes should reflect

• number of visible land and groove impressions,
• number of measurable land and groove impressions,
• measurements of useable land and groove impressions,
• measurements expressed to the nearest thousandth of an inch.

Dashed lines indicate damaged impressions; reliable measurements could not be taken.

Equipment

Using a comparison or stereomicroscope, land and groove impressions can be measured using a variety of equipment:

• Micrometer
• Calipers
• Glass measuring reticule
• Electronic reticule
• Digital measuring equipment and software
• MP 6 optical projector (no microscope)
• Machinists scale

Descriptions of each of the above equipment can be found in Module 7.

A technique used by many examiners for measuring the widths of land and groove impressions is the air gap method. This technique is fast and accurate, requiring only a calibrated microscope and a micrometer. Calipers can be substituted for the micrometer.

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