Guidelines for Barrel and Overall 

Length Measurements for Firearms Page 1 of 3 Adopted 9/26/13 

Guidelines for Barrel and Overall Length 

Measurements for Firearms

1.0 Objective 

The purpose of this document is to set forth guidelines to Firearms Examiners 

conducting barrel and overall length measurements for firearms. 

2.0 Introduction 

2.1 When handling any firearm, even for the purposes of measuring, safety is 

the first concern. Make sure the firearm is unloaded before conducting 

measurements. If there is any doubt about the operation of a firearm, 

consult with a qualified firearms instructor (if available), protocols or 

manufacturers’ literature before handling a firearm for measuring. 

2.2 When measuring barrel or overall length ensure the firearm is free from 

movement and stable for measuring, located in an area with proper lighting, 

and the proper calibration certificates are current and traceable to a National 

Institute Standards and Technology (NIST) standard. When a measurement 

is made where the uncertainty of that measurement will be reported, the 

calibration date and unique identifier for the device(s) used should be 

recorded in the examination record. 

2.3 Common equipment used to measure barrel and overall lengths are 

measuring tapes, steel rulers, measuring rods, and caliper devices. 

2.4 Measurements for altered barrels should include the longest measured 

length. 

3.0 Barrel Length Measurements 

3.1 Revolvers 

3.1.1 When measuring a revolver barrel, the distance parallel to the bore 

axis from the muzzle end to the farthest end of the forcing cone 

represents the length of the barrel. 

3.1.2 Ruler/Measuring Tape – a revolver barrel can be measured by placing 

a steel ruler/measuring tape on the exterior of the barrel, parallel to 

the axis of the bore to determine the barrel length. 

3.1.3 Measuring Rod - A measuring rod can be used to measure barrel 

length; however, it may be difficult to determine the starting point 

for the measuring rod. Consideration must be taken to determine 

how the starting point can be accurately achieved. A block at the 

muzzle or forcing cone end of the firearm, which is perpendicular to 

the axis of the bore can represent the starting point for the 

measuring rod to determine barrel length. 
Guidelines for Barrel and Overall 

Length Measurements for Firearms Page 2 of 3 Adopted 9/26/13 

3.1.4 Caliper – a revolver barrel can be measured by placing a caliper on 

the exterior of the barrel, parallel to the axis of the bore to determine 

the barrel length. 

3.2 Integral Chamber Barrels 

3.2.1 When measuring the barrel of a firearm that has an integral 

chamber, the distance parallel to the bore axis from the muzzle end 

to the breechface (with the action closed) represents the length of a 

barrel. 

3.2.2 Ruler/Measuring Tape – an integral chamber barrel can be measured 

by placing a steel ruler/measuring tape on the exterior of the barrel, 

parallel to the axis of the bore. 

3.2.3 When using a ruler/measuring tape to determine the barrel length 

measurement, read the increments perpendicular to the muzzle bore. 

3.2.4 Measuring rod - before measuring an integral chamber barrel, ensure 

that the firing pin is not impeding the measuring rod from making 

contact with the breechface. It may be necessary to cock the firearm 

to remove a protruding firing pin. In the case of a fixed firing pin, be 

certain it is not reducing the barrel length measurement. 

3.2.5 Ensure that the measuring rod, when inserted in the barrel, is 

parallel to the bore axis. 

3.2.6 All measurements are made perpendicular to the bore axis at the 

farthest point of the barrel. 

4.0 Overall Length Measurements 

4.1 When measuring the overall length of a firearm, the measurement is taken 

along a line which is parallel to the axis of the bore from a perpendicular 

tangential line which touches the rearmost point of the firearm to the 

muzzle. 

4.2 The overall firearm length is determined using a measuring platform or 

fixture that can achieve a right angle to the bore axis. A measuring platform 

or fixture is a device which has two points (jaws); one point is fixed and the 

other point articulates to determine the overall length. 
Guidelines for Barrel and Overall 

Length Measurements for Firearms Page 3 of 3 Adopted 9/26/13 

Appendix 1 – References

1. ATF National Firearms Act Handbook. U.S. Department of Justice Bureau of 

Alcohol, Tobacco, Firearms and Explosives Office of Enforcement Programs and 

Services [Internet]: ATFE-Publication 5320.8 [Revised April 2009]. Chapter 2. 

Available from: https://www.atf.gov/sites/default/files/assets/pdf-files/atf-p5320-8.pdf

Appendix 2 – Document History

Date Section Changes 

3/19/2013 Draft Approved 

3/20/2013 Posted online for review 

9/26/2013 Adopted