Proper way to measure barrel length

[spleify]

What is the proper way to measure barrel length?

I know a barrel on an AR can't be shorter than 16" so how do you measure the length? Is it an internal, or external measurement?

Thanks

[hobowh]

Rifle barrels are measured as overall length including the chamber, so measure from the base of the cartridge to the muzzle,

[spleify]

Rifle barrels are measured as overall length including the chamber, so measure from the base of the cartridge to the muzzle,

How is this done is what I would like to know. Official information would be much appreciated.

Thanks

[hobowh]

Drop a rod down the barrel (breech closed) mark it even with the muzzle measure the length of rod. Chamber is included in rifle barrel length.

[hobowh]

here is the gunwiki page
http://thegunwiki.com/Gunwiki/ProcMeasureBarrelLength

[jerrymrc]

Drop a rod down the barrel (breech closed) mark it even with the muzzle measure the length of rod. Chamber is included in rifle barrel length.

Yes, we have a winner.

[SAnd]

ATF P 5320.8
National Firearms Act Handbook
http://www.atf.gov/publications/firearms/nfa-handbook/

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. Permanent methods of attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver soldering, or blind pinning with the pin head welded over. Barrels 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 The rod is then marked at the furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and measured.

[spleify]

Thanks for the info folks. I was pretty sure this is how it was done, but I wanted and uninfluenced answer so I didnt mention anything.

Thanks so much!