"Interpolating" barrel length from photos

[bobsdad]

Do any of you ever try to determine actual barrel length directly from photos appearing on the auction sites (when barrel length is not shown in the description)? I was looking at a safety hammerless on a well known auction site recently. The barrel looked to be slightly longer (in the photo) than the common 3 1/4" for that model. But I wasn't sure, of course.
So I attempted to use a metal ruler to measure the length of the barrel and compare it to the length of the trigger guard (yes I'm talking about the photo on my computer screen!!). I then tried to use some proportionate calculations using the known length of the trigger guard for that model to determine the length of the barrel. The result I got was 3.5625 inches (something between 3 1/4" and 4"!!) two of the lengths offered for that model. So that method obviously did not work.
I know the best way would be just to ask the seller to measure the length, but I thought I'd try a more immediate (and complicated) method.
Am I wacko, or have any of you tried anything similar?

 

[mr.magnum]

i have to take measurements of things all day long in my line of work. because of this, i've started guessing lengths as a little game to entertain myself before measuring. you'd be amazed at how good you get when you do it often enough. it helps with barrels. esp if you are familiar with the firearm size. but not to the dgree you are talking. i prob coldn't catch a 3 3/4" k frame s&w unless there was a pic of another next to it, or i just noticed somethin funny about it.

 

[Jellybean]

If you know the length of the frame you can check to see how the length of the barrel compares to it.