Course LogoFirearm Examiner Training
Print Page Previous PageNext Page

Comparison Microscope

Home > Characterization and Evaluation of Fired Projectiles > General Riflling Characteristics > Equipment > Comparison Microscope
Viewed through the microscope, the opening (gap) between the end reference points of the micrometer aligned with the base of the fired bullet at the points of the beginning and end of the land impression and recorded in thousandths of an inch.

Air gap meaurement under
a comparison microscope

Courtesy of Ronnie Freels

Click for larger image

The air gap method can be used to measure land and groove impressions.

Using a comparison microscope:

  1. Place a fired evidence bullet on a bullet mount on one stage of the comparison microscope.
  2. Mount the micrometer, jaws up, on the other stage.
  3. Set the objective lenses for both stages to the same magnification.
  4. Focus both lenses.
  5. Open the jaws of the micrometer to match the reference points on the fired bullet.
  6. Record reading (air gap) to the nearest thousandth of an inch.
  7. Rotate the bullet to the next land or groove impression.
  8. Repeat steps 4 - 7 until all land and groove impressions are measured.

< Previous Page  ::  Next Page >

Association of Firearm and Tool Mark Examiners logo
Submit Change Request