- 1). Place the fire-formed brass casing, which is the spent brass casing from a cartridge previously fired in the firearm, in the base of the precision micrometer, with the primer end first.
- 2). Screw the top of the precision micrometer down onto the lower half until it makes contact with the brass casing. Apply slight pressure so that it is screwed tight, but be careful not to apply excessive force as this will yield an inaccurate reading.
- 3). Check the reading on the side of the precision micrometer. The marks on the lower half of the micrometer are in 5/100 inch increments, while the marks on the top half of the micrometer are in 2/100 inch increments. Add the two numbers together to get a total headspace reading. Compare the headspace to the recommended length for the cartridge to determine if the case needs to be altered.
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