long guns & most handguns

Barrel length is the distance between the closed bolt face, or breech face, to the furthermost end of the barrel or permanently attached muzzle device.

revolvers

Barrel length is the overall length of the barrel. It does not include the length of the barrel-cylinder gap or chamber.

While there are different methods for measuring a barrel, this is how BATFE does it (rifles &ﾠshotguns) under the National Firearms Act.

“The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to the furthermost end of the barrel or permanently attached muzzle device. ﾠ. . .ﾠﾠBarrels are measured by inserting a dowel rod into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and measured.”

 

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.

ATF National Firearms Act Handbook. Washington, DC: U.S. Dept. of Justice, Bureau of Alcohol, Tobacco, Firearms and Explosives, Office of Enforcement Programs and Services, 2009. Print.ﾠ[available for free .PDF download]