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