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ATF Standard Method to Measure Barrel Lengths
This is ATF's standard method for measuring the length of gun barrels - it has been documented to be the case for shotguns and rifles. I haven't found anything stating that this is the standard method for handguns, but technically it should be fine.
In practice, unless you're pushing really close to a minimum length limit, you could just use a cleaning rod in place of a dowel. But if you're close, you should get a rod of the same diameter as the barrel. On shotguns especially, it's possible for a slightly-angled rod to measure a longer distance than a rod of the correct diameter.
-- SeanNewton - 24 Jan 2008
- Unload the weapon.
- Close the action.
- Cock the action (this is important - unless cocked, the firing pin will protrude from the bolt face and may not provide the correct measurement.
- Run a dowel rod of approximately the bore diameter, down the barrel.
- When the rod is down as far as it will go, mark the spot where the barrel and any permanently mounted attachments end on the rod. Screwed-on attachments do not count.
- Measure the length of the rod between the mark you just made, and the end which you ran down the barrel.
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