How To Accurately Measure A Gun Barrel's Length

Above, the Winchester Model 12 shotgun barrels came in 26
to 30 inches with some at 32 inches. Photo by Armand Vaquer.

Here's something that I haven't given any thought to. I generally relied on the gun's specs in the advertisement or the gun's owner's manual.

That is: How to accurately measure a gun barrel's length?  

The NRA's American Rifleman website answers that question for measuring.

This is the recommended method (except revolvers), here's what they said:

Rather than debating your math and measurements, I think that I can clarify your questions by referring to the recommended methods used by the Sporting Arms and Ammunition Institute (SAAMI) and the ATF. Both use a standard method for measuring barrels in firearms with chambers manufactured as an integral part of the barrel. The procedure is easy to follow.

When performing this procedure, safety is of the highest concern. Remove any and all ammunition from the work area to mitigate the possibility of a mistake. Once the gun is verified clear and empty, close the action as though it were ready to be fired.

Insert a wooden dowel, cleaning rod or other suitable object into the muzzle end of the barrel until it comes to a rest against the breech face where the firing pin protrudes through when firing. Once the dowel is resting firmly against the breech face, mark it at the muzzle with a magic marker or similar marking medium. Next withdraw the dowel and measure the distance between the mark on the dowel and the end that was resting against the breech face.

That measurement is considered the industry standard for pistols, rifles, shotguns or any other type of firearm having a chamber integral with the barrel (Revolvers are treated differently because they have multiple chambers not integral with the barrel).

To read more (especially for revolvers), go here.