Forensics
Lab Activity
Using a Vernier Caliper to Measure “Brass”
Name ____________________
Per ____ Date _____________
When a bullet casing (commonly called “brass”) is found at a crime scene, someone has to
measure it to determine what caliber it is. This measurement is often done using a vernier
caliper. Once measured in centimeters using a
Caliber: The diameter of a projectile or the bore of
vernier caliper, the measurement is converted to
a firearm; expressed in hundredths of an inch
(example .30 cal.) in the United States and in
either inches (to determine US caliber) or
metric millimeters (example 5.56mm) in Europe,
millimeters (to determine metric caliber).
Asia, etc.
US caliber is expressed in hundredths of an inch (.30 cal is read as “30 caliber”) while
metric caliber is expressed in millimeters (9mm is read as “nine millimeter”).
In this activity, you will first practice converting from centimeters to inches and
millimeters. Once proficient in converting, you will be presented with a collection of empty
(unprimed) brass to measure to determine caliber.
Converting from Centimeters to Inches
Given that 2.54 cm = 1 inch, one simply needs to divide a measurement in centimeters by
2.54 to convert to inches. In ballistics, such conversions are rounded to 2 decimal places.
Ex: to convert a measurement of 0.46cm to US caliber…
0.46cm
.
in ⇒ .18 cal
= 01811
2.54cm / in
Practice on this conversion process is offered in Table 1. Convert each of the centimeter
measurements shown in Column 1 to inches (Column 2; show 4 decimal places) and then to
US caliber (Column 3).
Table 1
Measurement (cm)
0.22
0.36
0.42
0.77
0.85
inches
US caliber
Converting from Centimeters to Millimeters
As previously practiced, converting from centimeters to millimeters is simply a matter of
shifting the decimal place one position to the right.
Ex: to convert a measurement of 0.46cm to metric caliber… 0.46 cm = 4.6 mm
Practice on this conversion process is offered in Table 2. Convert each of the centimeter
measurements shown in Column 1 to metric caliber (Column 2).
Table 2
Measurement (cm)
0.22
metric caliber
0.43
0.57
Table 3 below lists some common calibers. Convert the given data to complete the table.
Table 3
US caliber
.17
metric caliber
.22
.223
.25
.30
.357
.38
.45
.50
7
7.65
9
10
Forensics
Lab Activity
Using a Vernier Caliper to Measure “Brass”
Name _____________________
Per ____ Date ______________
Measuring Brass
Obtain a set of brass samples from your instructor.
For each lettered brass (there should be two samples of each)
Measure the inside diameter (ID) of the mouth.
Rotate the brass 90o
Measure the inside diameter of the mouth again.
Repeat steps 1 – 3 for the second sample.
Average the four measurements.
Convert the average to both inches and millimeters.
Using the information in Table 3, determine the caliber of the sample.
Repeat steps 1 – 7 for all lettered samples.
Sample A
Trial
Who
ID (cm)
1
2
3
4
Sample B
Trial
ave
in
mm
convert to
caliber
Sample C
Trial
Who
1
2
3
4
in
mm
caliber
ID (cm)
Sample D
Trial
Who
1
2
3
4
ave
convert to
ID (cm)
1
2
3
4
ave
convert to
Who
in
mm
caliber
ave
convert to
in
mm
caliber
ID (cm)
Sample E
Trial
Who
ID (cm)
1
2
3
4
Sample F
Trial
ave
in
mm
convert to
caliber
Sample G
Trial
Who
ID (cm)
Sample H
Trial
ID (cm)
ave
in
mm
convert to
caliber
Who
1
2
3
4
in
mm
caliber
ID (cm)
Sample J
Trial
Who
1
2
3
4
ave
convert to
Who
1
2
3
4
ave
Sample I
Trial
in
mm
caliber
1
2
3
4
convert to
ID (cm)
1
2
3
4
ave
convert to
Who
in
mm
caliber
ave
convert to
in
mm
caliber
ID (cm)