I have seen people who measure barrel length incorrectly.  Barrel length includes the part that is threaded into the action, correct?  I was watching Pawn Stars and a fellow brought in an 1886 Winchester.  Their gun expert was called in and measured the barrel only to the action not including the threads.  He said the barrel had been shortened 3/4 of an inch and they knocked down the price.  I think the gun expert is not that much of an expert.  You can bet when they sell it they will not tell the buyer that the barrel was shortened.  Just a comment after reading a post .

tdelewis,
Seems like Pawn Stars measured only to the receiver
and then said it was shortened 3/4".
It may NOT have been shortened since the barrel goes into the receiver about that much.
So they could turn around and sell it as not cut....
beltfed/arnie

Yup.  Bbl length is measured from bolt-face to muzzle.  (unless I'm wrong, but that has NEVER happened before)  Further, I NEVER exadurate.

I remember a Pope for sale at Julia Action and the expert had labeled the Muzzle Loading Rod as a "Wiping Rod" actually have seen that more than a couple of times. Regards, FITZ.

I'd say the pawnbroker was an expert at screwing the owner of the rifle by faking the true barrel length.

"Expert" is a pretty loose term. Most “experts” are very knowledgeable about a very small slice of their field. For example most auction “experts” are pretty good with commercial firearms if it came out of a factory some body wrote a book and they can look it up. They don’t do so well with custom made target firearms. Most competitors have little to no regard for factory specs. If a modification helps them they put it on and don’t care that it isn’t factory correct. Most auction don’t understand or care about shooter collectors they will break up rifles and their shooting kits in a heartbeat to get more money for the seller. That is how so many Pope false muzzles have gotten separated from their rifles over the years the rifle gets sold on the first day and the kit with the false muzzle re&de mold and range book get sold two days later as Assorted reloading tools. The auction houses mostly wouldn’t know Pope from Parkenfarker by site. 

40 Rod

Rifles, shotguns, pistols, from the bolt face, but revolvers from the front of the cylinder, if I am not mistaken...

How to measure bra size... oops, wrong one. 

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.
Procedure
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.
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.